【答案】Both his parents look sad . Maybe they __what's happened to him .

Both his parents look sad . Maybe they __what's happened to him .

A. knew

B. have known

C. must know

D.will know

【答案】

B

【解析】

句意：他的父母看起来都伤心。也许他们知道了他发生了什么。Ａ. knew知道（过去） ，Ｂ. have known（已经）知道了，Ｃ. must know 一定知道，Ｄ.will know将会知道。据句意have known表示known发生在过去对现在造成的影响是Both his parents look sad .，句子要用现在完成时态，故选B。