— I dropped in at your house at about ten last night, but you weren't in. 

— I ______ regular exercises at the club. [ ]

A. did                        

B. was doing

C. had done                  

D. have been doing

对线性控制阀来说，DCS操作面板手操器输出50%信号给现场执行机构则控制阀相应开度是（）。

A、25%

B、50%

C、75%

D、100%