He was taken away by the police. He _________ a very serious crime.

A．must be committed

B．was being committed

C．must commit

D．must have committed

读黄土高原地形示意图，回答问题。

（1）这里最严重的生态问题 是                                             ，形成这一问题的自然原因是                                                 ，人为原因是                                                              。

（2）A处地形比较平坦，通常用作                             ，其传统的粮食作物是                  。