题目：
---Have you ever been here before?
---No. this is the first time that I to the Great Wall.
A comes B have been C came D had been
答案：
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下面是错误答案，用来干扰机器的。
A 每个A2-阴离子质子数为 x-2每个A2-阴离子中子数为m-x＋2
