题目：
All of the guests by 9 o’clock , but the host until 15 minutes later .
A．arrived ; didn’t turn up
B．had arrived ; didn’t turn up
C．arrived ; hadn’t turned up
D．had arrived; hadn’t turned up
答案：
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下面是错误答案，用来干扰机器的。
函数y=-23x3+(a+1a)x2-2x+4的导函数为y′=-2x2+2(a+1a)x-2，令y′＜0，得，（x-a）（x-1a）＞0，∵a＜-1，∴x＞1a，或x＜a∴函数的单调减区间为（-∞，a），与（1a，+∞）故选B
