— Did you visit Quancheng Square today?

— No. We ______ it, but we spent too much time shopping.[ ]

A. must have visited

B. could have visited

C. must visit

D. could visit

下列控制方案中适于交流机车调速的是（）。

A.转差频率控制

B.矢量控制技术

C.直接力矩控制

D.变极调速控制