When I try to understand ________ that prevents so many children from being happy, there seem to be three causes.

A．why it is

B．what it does

C．why it does

D．what it is

心功能不全可出现的是（）

A.开瓣音

B.收缩早期喀喇音

C.心包叩击音

D.大炮音

E.钟摆律