_ is mentioned above, a lot of work had been done to create man-made materials.

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问题 _ is mentioned above, a lot of work had been done to create man-made materials.

选项 A、That
B、Whether
C、As
D、if

答案C

解析 本题为语境词汇题,这里需要一个单词表达“如上所述”的意思来指导前面所做的表述。唯有as引导的非限制性定语从句才能做到,关系代词as在句中作主语。所以答案为C)。
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