--________do you go to the dentist?

--Let me see, twice a year, maybe.

A．How many

B．How much

C．How often

D．How soon

下列可引起混合性呼吸困难的是（）

A.气管异物

B.白喉

C.大面积肺梗死

D.支气管哮喘

E.慢性阻塞性肺气肿