He ______ many books in the last few years. [ ]

A. wrote

B. has written

C. was writing

D. was writing

选择萃取剂将碘水中的碘萃取出来，这种萃取剂应具备的性质是（　　）B．

A．不溶于水，且必须不与碘发生化学反应

B．不溶于水，且比水更容易使碘溶解

C．不溶于水，且必须比水密度大

D．不溶于水，且必须比水密度小