I expect B is the answer you're looking for; however, D is also so close. The problem I have with B is that there is no rainbow in medium to heavy rain. Rainbows are common just AFTER a rain and there is still a mist in the sky and that's why I like D. Frankly, I don't like questions like this because it forces students to pick the "lesser of two evils". It is good to make a student aware of details and small differences but not, in my opinion, to see the nuance in a sentence as a distinguishing feature. Others may disagree. For example, I have seen rainbows while flying.It is not uncommon. In all instances when I was looking we were NOT in rain but looking in the distance at a misty view. Have you seen a double rainbow; i.e., one above the other? Cool.