Malaysia Airlines Flight 370 ___________ course and turned back toward Kuala Lumpur -- but there were no indications from the pilot that he was doing so.

A．must have changed

B．could have changed

C．might have changed

D．should have changed.

中国网民的年龄结构偏低，大多是（）下的人。

A.35

B.40

C.45

D.50