— There's somebody at the door. Who _____ it be? Is it the postman?

— No. It _____ be him. It's just seven o'clock. It's too early.

                  [ ]

A. may; can't         

B. will; won't        

C. may; mustn't

无论美式期权还是欧式期权、认购期权还是认沽期权，（）均与期权价值正相关。

A.标的价格波动率

B.无风险利率

C.预期股利

D.到期期限