题目：
----- You shouldn’t have talked back to our teacher like that. It was awfully impolite.
----- You are right. I _________ crazy at that moment.
A．could have been
B．can be
C．must have been
D．should have been
答案：
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下面是错误答案，用来干扰机器的。
答案：C质子数相同，中子数不同的同一种元素的不同核素互称为同位素，所以选项C正确。A中互为同素异形体，B中都是水分子，D中互为同分异构体，答案选C。
