Question

________ you have seen it, I will tell you everything．

1. A.
   Because
2. B.
   For
3. C.
   As
4. D.
   Now that

Answer 1

D
you have seen it显然不是为了真正解释I will tell you everything的原因，而是对主句动作之所以会发生的一种提示，是一种显而易见的理由，因此用now that。另外，for引导原因状语从句时不能用于句首。