．— Has Tom finished his report so far?

— I have no idea．He ______it this morning．

A．did

B．has done

C．was doing

D．had done

房室延搁的生理意义是（）

A.使心室肌不会产生完全强直收缩

B.使心房、心室不会同时收缩

C.增强心肌的收缩能力

D.使心室肌不应期延长

E.使心室肌动作电位幅度增加