— I heard they went skating in the mountains last winter.

— It _____ true because there was little snow there.

A．may not be

B．won’t be

C．couldn’t

D．mustn’t be

下述哪项不是主动脉瓣狭窄的主要临床表现（）

A.栓塞

B.心绞痛

C.晕厥

D.左心功能不全

E.猝死