They considered___ a computer, which was considered ___ a great help in their work.

A．to buy；to be

B．buying；being

C．to buy；being

D．buying；to be

脊髓灰质炎活疫苗现场试验结果表明：接种疫苗组儿童脊髓灰质炎的发病率是16／10万，接受安慰剂组儿童的发病率是57／10万，因此该疫苗的效果指数是（）。

A.72%

B.0.28

C.72

D.3.6

E.41