I will と I'm going to の違い

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A: We don't have any bread.
B: Yes, I know. I _____ some. I took some money from your purse.

will buy
am going to buy
buy
have bought

A: Did you call Sarah?
B: Oh, I forgot! Thanks for reminding me. I _____ her now.

am going to call
will call
have called
calling

A: Why are you taking a coat?
B: Because I think it _____ cold later.

will be
am going to be
is being
has been

A: Look at those dark clouds!
B: Yes, I think it _____ soon.

is going to rain
will rain
rains
has rained