Question

Explain the purpose of the apostrophe in each word, and explain what the apostrophe is probably replacing. If there is more than one possibility, try to get all possibilities and explain each.
 
1. It's

2. Sue's
 
3. D'Giorno
 
4. We'll
 
5. '70s
 
6. Bill's
 
7. 'til
 
8. They're
 
9. Aren't
 
10. O'Reilly
 

Answer 1

except for 2 and 6 (which indicate possession), the apostrophes indicate a missing letter.

Even 2 and 6 can mean that, if you meant to say
Sue's going to sue me.
short for "Sue is going to sue me."

Answer 2

oobleck is right.

Plus, #5 involves missing numerals -- '70s means 1970s.