before doing与before do

问题描述:

before doing与before do
What do you do to the prawns before you_____them?
A.frozen.B .froze.C.freeze.D.freezing
求解答〜〜〜

freeze,前后时态必须一致,都是一般现在时请问什么时候是doing呢〜what are you doing to the prawns before you freeze them?