I was on the highway when his car went past, followed by a police car. They _____ at least 150km

an hour.[ ]

A. should have been doing

B. would have done

C. could have done

D. must have been doing

我国企业开始涉足电子商务于（）

A.1991年

B.1992年

C.1993年

D.1994年