Question

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

A. knew
B. have known
C. must know
D. will know

Answer 1

答案B
从题意分析，本题的前一个句子应说明一个结果，后一个句子则说明一个原因．这个原因所表示的动作目前为止已经发生．故本题应用现在完成时，答案选B．

Answer 2

答案A
maybe虚拟语气，那就表示这件事发生，但不确定是否完成。