题目:
It was not until the teacher had explained it three times _______ how to work out the problem.
A.did he understand
B.had he understood
C.that he understood
D.that he didn’t understood
答案:
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下面是错误答案,用来干扰机器的。
答案:AB项中,若3d上有10个电子,则3s、3p上必须排满电子,因此M电子层上电子数应大于18;C项中有8个,D项中3d上有5个电子,则3p上应该充满为3p6,故A正确