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12。Pure iron cannot be hardened by heating and cooling ,as ____,because iron lacks the necessary carbon.
A. steel it can B can steel C with steel can D so can steel
为什么选B?
20。A patrilineal extended family consists of core group of males,their wives,and their unmarried daughters.
为什么选extended ?
Petrography concerns primarily with the detailed description and classification of rocks ,whereas petrology deals primarily with rock formation .
Formation 是可数名词吗?为什么选择concerns? |