Question

Since computers were introduced, they_________tobe useful tools for people weak in math.

A.hadproved
B.haveproved
C.willprove
D.areproving

Answer 1

【答案】：B
句意：自电脑问世以后，它们已被证明是一种对数学薄弱的人们有用的工具。since引导的从句中的动词用一般过去时，主句用现在完成时，故选B。