—I called you at half past nine this morning but there was no answer.

—Oh, sorry. I ______ with my cousin in the supermarket. [ ]

A. shop

B. was shopping

C. shopped

D. will shop

真正制约地下铁路建设的因素是( )问题。

A．城市交通流量

B．城市人口

C．技术

D．经济性