It was not until the 1990s ________ industrial robots became really cheap, and t

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问题 It was not until the 1990s ________ industrial robots became really cheap, and therefore a possible option.

选项 A、which
B、who
C、whom
D、that

答案 D

解析根据语法，空格处应填入that构成强调句型。故选D。

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