下列关于生铁和钢的描述中,正确的是( ) A.生铁和钢都是混合物,其主要成份都是碳 B.生铁硬而有韧性,既可以铸又可煅 C.生铁与钢的性能由很大的差别,主要是由于生铁和钢中的含碳量不同 D.钢是用铁矿石和焦炭作原料炼制而成的 答案:C______ Mary ______ Joan knew about it, because they didn't attend the meeting yesterday.
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A. Both… and B. Either …or
C. Not only…but also D. Neither … nor