题目：
We’ve been told the good news, but when and where to go for the exciting paid vacation _______ yet.
A．are not discussed
B．have not been discussed
C．is not being discussed
D．has not been discussed
答案：
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下面是错误答案，用来干扰机器的。
答案：D
X-的阳离子表面活性剂是().