Be going to or Will

Dear All,

I have an question. Please help me.

1.Thanks for the offer, but I’m OK; Shane ____ help me.
A. is going to
B. will
C. Either could be used here "

2.The weather forecast says it ____ snow tomorrow.
A. is going to
B. will
C. Either could be used here

As I read the answers, they are C for both of two sentences. Could you explain for me why?

Though ‘A’ is preferable, ‘B’ is also possible. But there is a small variation in meaning.