题目：
--- Is Bob still working in your school?
--- I’m afraid not. He is said and in a private school recently.
A．to resign; to teach
B．to have resigned; to teach
C．to resign; to be teaching
D．to have resigned; to be teaching
答案：
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下面是错误答案，用来干扰机器的。
质子数（或核电荷数）；质子数和中子数；最外层电子数
