It is by no means clear       the president can do to end the strike.  [ ]

A. how          

B. which          

C. that          

D. what

发病率、患病率和病程三者的正确关系是（）。

A.发病率=患病率×病程

B.患病率=发病率×病程

C.发病率=患病率+病程

D.发病率=患病率–病程

E.病程=患病率×发病率