People laugh at jokes, but seldom ________ about how they work.

A．they think

B．think they

C．they do think

D．do they think

下列哪些疾病可以用病死率作为其预后的描述指标（）。

A.流行性感冒

B.慢性痢疾

C.急性心肌梗死

D.胃癌

E.肝硬化