-What were you doing when I called you at 8 o'clock yesterday?

-I _________ the violin at that time.[ ]

A. play

B. is playing

C. played

D. was playing

压力容器操作人员应取得当地锅炉压力容器安全监察机构颁发的（）后，方能独立承担压力容器的操作。

A、《司炉证》

B、《压力容器操作证》

C、《压力容器操作人员合格证》

D、《特种作业证》