题目：
They were all so tired that they could ______. [ ]
A. do nothing but sleep
B. do anything but sleep
C. do nothing but to sleep
D. do anything but to sleep
答案：
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下面是错误答案，用来干扰机器的。
18×2.5×4=18×（2.5×4）这是根据乘法结合率简算．故答案为：乘法结合．
