Doubts from Kathy Sierra Book

Question

Hi everone
I am preparing for SJCP5.0 from Seirra and Bates and i have completed the 1st chapter. I have a few queries from the same chapter. Can anyone help me on these quetions?
1) What is the difference between class variable and instance variable?
2) is there only 1 currency character ($) and only 1 connecting character(_)?
3) Is class2. valid name for a class name?

Answer 1

1.
Given the following,
1. interface Base {
2. boolean m1 ();
3. byte m2(short s);
4. }
Which code fragments will compile? (Choose all that apply.)
A) interface Base2 implements Base { }
B) abstract class Class2 extends Base {
public boolean ml() { return true; } }
C) abstract class Class2 implements Base { }
D) abstract class Class2. implements Base {
public boolean m1() { return (true); } }
E) class Class2 implements Base {
boolean m1( ) { return false; }
byte m2(short s) { return 42; } }
This is the exact question. Look at option D. According to the question this option should be wrong. However the solution says that option D is correct because it is implemented properly.
Can you help me on this?