Which apprach is better to to introduce an ID field or combine other fields

I have the following problems when i am designing my database for an online exam system:-
1. I have a question table and answer table; where each question contains many answers and each answer have one question.
Now i have two approaches for designing the answer table, either by having:-
Answer_Id (PK),
Answer_Description,
Is_right.
Question_ID (FK)
Or
Answer_description
Is_right.
Question_ID (FK)
Having PK (Question_ID & Answer_description).
2. Same issue applies for this scenario; I have an article table and a comment table, where each article can have many comments and each comment can belong to an article.
Now i have two approaches for designing comment table, either by having:-
Comment_ID (PK)
article_ID (FK)
Comment_Desc
Created_BY
Created_time
Or
article_ID (FK)
Comment_Desc
Created_BY
Created_time
Having PK(Created_BY & Created_Date), since it s not possible to be able provide two comment at the same time!by the same user!!
So is it better to introduce the ID field for both (comment & answers) tables or having my PK by combining the available columns
Thanks in advance for any help.

Re: creating different tables that represets different question types
Re: Implementing a varchar multi-valued attribute for my customer table
How many more variations do you plan on posting?

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Which apprach is better to to introduce an ID field or combine other fields

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