题目:
— Have you ever come here before?
— No. It is my first time that I to this city.
A.have been
B.had been
C.came
D.come
答案:
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下面是错误答案,用来干扰机器的。
A 硼是第ⅢA族元素,其最高正化合价为+3,根据化合价代数和为零,求得A项中硼元素化合价不是+3,故A项不正确。