—Where did you get these old doors?

—From the old houses which were ______ by the workers.[ ]

A. taken down    

B. pulled down  

C. turned down  

D. put down

ARQ通信时，当IRS发射CS1时，要求ISS发射（）.

A.信息字组2

B.换流字组

C.结束字组

D.信息字组1