________the problem, he wouldn't have committed those mistakes.

A．If he understood

B．Had he understood

C．When he had understood

D．If he would understood

急性有机磷农药中毒最主要的死因是（）。

A.呼吸衰竭

B.中毒性心肌炎

C.中毒性休克

D.急性肾功能衰竭

E.电解质、酸碱平衡紊乱