— Has Sam finished his homework today?　　

— I have no idea. He ________ it this morning.　　

A．did

B．was doing

C．has done

D．had done

犬产后低血钙症的典型症状是（）。

A.腹泻

B.排尿困难

C.肌肉痉挛

D.血红蛋白尿

E.呼吸困难