Since computers were introduced，they（）to
单项选择题 【2019年真题】Since computers were introduced，they（）to be useful tools for people weak in math.
A.had proved
B.have proved
C.will prove
D.are proving
正确答案：B
答案解析: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。
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