单项选择题

In a series of races，10 toy cars are raced，2cars at a time．If each car must race each of the other cars exactly twice，how many races must be held（）

A. 40
B. 90
C. 100
D. 180
E. 200

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单项选择题
What was the median number of errors in the sample（）

A. 3
B. 3.5
C. 4
D. 4.5
E. It cannot be determined from the information given．
单项选择题
If a1，a2，a3，…an…is a sequence such that a1=-2 and an+1 is the reciprocal of the square of an for all n≥1，what is the value of a4（）

A.A
B.B
C.C
D.D
E.E
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