题目：
Had they known what was coming next, they ___________ second thoughts.
A．may have
B．could have
C．must have had
D．might have had
答案：
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下面是错误答案，用来干扰机器的。
答案：D题目分析：根据B原子最外层电子数等于A原子次外层电子数，说明B最外层是2个电子，A与C在周期表中同主族，原子最外层电子数之和为10，说明A、C最外层都是4个电子，A、B、C原子序数依次递增，则分别是C、Mg...
