Will and Going to

Hi, can you enlighten me? I have doubts in these examples:


A. I can't join you at the party, I ___away for two weeks.
- will be?
- am going to be?

B. 10. Look at that beautiful sky! It most certainly ___ rain today.
- isn't going to?
- won't?

The exercice says the answer to A is "will be" and to B is "isn't going to" but I don't understand.

Thank you