A. It's on January 10^th.     

B. Because my birthday is in January.

C. When is her birthday? 

D. My mother's birthday is in January, too.

Mary: January is a special month.

Tom: Why?

Mary:

Tom: When is your birthday?

Mary:And my grandmother's birthday is on January 5^th.

Tom: That's fine.

Mary:

Tom: 

Mary: It's on January 18^th.