Need help on fixing this bug

Question

Hello!
I have my cod which needs some bug fixing. This is a drag and drop application where the user will drap the correct answer on corresponding targets. My problem here is, when one target is already occupied by an object i can still drop another object there, which shouldn't. can you please help me fix this?
thanks in advance!
Sincerely.
Milo
Here's my code:
var startX: Number;
var startY: Number;
var correct: Number = 0;
var attempt: Number = 0;
var currentlyDragged:MovieClip;
// collection of objects stored in array
// so that you can reference them programmatically
var objects:Array = [at1, in1, in2, in3, in4, in5, in6, in7, in8, on1, on2];
activateObjects();
// assigns listeners and other functionality to the objects in objects array
function activateObjects():void {
     for each(var mc:MovieClip in objects) {
          mc.addEventListener(MouseEvent.MOUSE_DOWN, pickObject);
          mc.buttonMode = true;
          // assign drop targets based on names
          switch(String(mc.name).substring(0, 2)) {
               case "at":
                    mc.dropTargets = [targetAT1];
               break;
               case "in":
                    mc.dropTargets = [targetIN1, targetIN2, targetIN3, targetIN4, targetIN5, targetIN6, targetIN7, targetIN8];
               break;
               case "on":
                    mc.dropTargets = [targetON1, targetON2];
               break;
function pickObject(e:MouseEvent):void {
     currentlyDragged = MovieClip(e.currentTarget);
     currentlyDragged.startDrag();
     startX = currentlyDragged.x;
     startY = currentlyDragged.y;
     stage.addEventListener(MouseEvent.MOUSE_UP, dropObject);
function dropObject(e:MouseEvent):void {
     stage.removeEventListener(MouseEvent.MOUSE_UP, dropObject);
     stopDrag();
     var droppedOn:MovieClip;
     if (currentlyDragged.dropTarget) {
          // loop through targets belonging to the currently dragged clip
          for each(var mc:MovieClip in currentlyDragged.dropTargets) {
               if (currentlyDragged.hitTestObject(mc)) {
                    // get the target
                    droppedOn = mc;
                    currentlyDragged.removeEventListener(MouseEvent.MOUSE_DOWN, pickObject);
                    currentlyDragged.buttonMode = false;
                    currentlyDragged.x = droppedOn.x;
                    currentlyDragged.y = droppedOn.y;
                    correct++;
                    correctCounter.text = String(correct);
                    // stop loop - it is not necessary to continue
                    break;
     attempt++;
     attemptCounter.text = String(attempt);
     // return to the initial position if there is no hit
     if (!droppedOn) {
        currentlyDragged.x = startX;
        currentlyDragged.y = startY;
     if (correct == objects.length) {
          var congrats:CongratsMC = new CongratsMC();
          // place i in the middle of the screen
          congrats.x = (stage.stageWidth - congrats.width) * .5;
          congrats.y = (stage.stageHeight - congrats.height) * .5;
          addChild(congrats);

Answer 1

Try the code below.
Also, use int instead of Number whenever possible - it is smaller and faster.
import flash.display.MovieClip;
import flash.display.Sprite;
var startX:Number;
var startY:Number;
var correct:int = 0;
var attempt:int = 0;
var currentlyDragged:MovieClip;
// collection of objects stored in array
// so that you can reference them programmatically
var objects:Array = [at1, in1, in2, in3, in4, in5, in6, in7, in8, on1, on2];
var congrats = new CongratsMC();
congrats.addEventListener("close", closeCongrats);
activateObjects();
// assigns listeners and other functionality to the objects in objects array
function activateObjects():void {
     for each(var mc:MovieClip in objects) {
          mc.addEventListener(MouseEvent.MOUSE_DOWN, pickObject);
          mc.buttonMode = true;
          // assign drop targets based on names
          switch(String(mc.name).substring(0, 2)) {
               case "at":
                    mc.dropTargets = [targetAT1];
               break;
               case "in":
                    mc.dropTargets = [targetIN1, targetIN2, targetIN3, targetIN4, targetIN5, targetIN6, targetIN7, targetIN8];
               break;
               case "on":
                    mc.dropTargets = [targetON1, targetON2];
               break;
function pickObject(e:MouseEvent):void {
     currentlyDragged = MovieClip(e.currentTarget);
     currentlyDragged.startDrag();
     startX = currentlyDragged.x;
     startY = currentlyDragged.y;
     stage.addEventListener(MouseEvent.MOUSE_UP, dropObject);
function dropObject(e:MouseEvent):void {
     stage.removeEventListener(MouseEvent.MOUSE_UP, dropObject);
     stopDrag();
     var droppedOn:MovieClip;
     correctCounter.text = String(correct);
     if (currentlyDragged.dropTarget) {
          // loop through targets belonging to the currently dragged clip
          for each(var mc:MovieClip in currentlyDragged.dropTargets) {
               if (currentlyDragged.hitTestObject(mc)&& mc.notUsed) {
                    // get the target
                    droppedOn = mc;
                    mc.notUsed = false;
                    currentlyDragged.removeEventListener(MouseEvent.MOUSE_DOWN, pickObject);
                    currentlyDragged.buttonMode = false;
                    currentlyDragged.x = droppedOn.x;
                    currentlyDragged.y = droppedOn.y;
                    correct++;
                    correctCounter.text = String(correct);
                    removeTarget(droppedOn);
                    // stop loop - it is not necessary to continue
                    break;
     attempt++;
     attemptCounter.text = String(attempt);
     // return to the initial position if there is no hit
     if (!droppedOn) {
        currentlyDragged.x = startX;
        currentlyDragged.y = startY;
     if (correct == objects.length) {
          // place i in the middle of the screen
          congrats.x = (stage.stageWidth - congrats.width) * .5;
          congrats.y = (stage.stageHeight - congrats.height) * .5;
          addChild(congrats);
function removeTarget(target:Sprite):void {
     var i:int = 0;
     for each(var mc:MovieClip in objects) {
          for (i = 0; i < mc.dropTargets.length; i++) {
               if (mc.dropTargets[i] == target) {
                    mc.dropTargets.splice(i, 1);
function closeCongrats(e:Event):void {
     removeChild(congrats);