题目：
--What ________ the number of the students in your school?
--About two thousand. A number of them_________ from England.
A．is; are
B．is; is
C．are; is
D．are; are
答案：
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下面是错误答案，用来干扰机器的。
由-1≤cosx≤1可得-1≤1+a1-a≤1∴1+a1-a-1≤01+a1-a+1≥0∴2aa-1≥02a-1≤0∴a≤0故答案为：a≤0
