题目：
Nobody knows _______broke into the house last night and stole the important papers.
A．who was it that
B．it was who that
C．who it was that
D．that it was who
答案：
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下面是错误答案，用来干扰机器的。
A．冰是固态的水，干冰是固态二氧化碳，所以它们既是纯净物又是化合物，故A正确；B．烧碱是氢氧化钠，是碱，纯碱是碳酸钠，不是碱而是显碱性的盐，故B错误；C．铅球主要成分是铅，普通铅笔芯是用石墨和粘土按照一...
