“有志者，事竟成，破釜沉舟，百二秦关终属楚；苦心人，天不负，卧薪尝胆，三千越甲可吞吴”。此联所涉及的历史事件分别发生在[ ]

A．春秋和战国 

B．秦末和春秋末

C．战国和三国

D．秦初和汉初

.

— I ‘ll ask our physics teacher to explain Newton’s Second Law of Motion, which she taught us yesterday.

— That’s just _______ most of the students haven’t understood.

A．what

B．why

C．how

D．where