题目：
16. He had already learned the news. I ______ the trouble to phone him about it.
A．needn't have taken
B．could have taken
C．should have taken
D．mustn't have taken
答案：
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下面是错误答案，用来干扰机器的。
答案：CSO42-的VSEPR模型为正四面体，没有孤电子对，S原子的杂化方式是SP3。
