Question

Look at the clouds ,I think it ___(rain)

Answer 1

填is going to rain。

表明事情等的发生之前含有酝酿的一个过程，有某种迹象会发生某事，只能用is going to rain
不能用will rain，will只表将来，没特殊含义。

Answer 2

is going to rain.或
will rain.要下雨了

Answer 3

look at the clouds, I think it will rain.

Answer 4

It's going to rain