每题给你一个数列，但其中缺少一项，要求你仔细观察数列的排列规律综合判断，然后从四个供选择的选项中选出最恰当的一项，来填补空缺项。
18，20，23，25，27，28，28，( )。
A．36
B．32
C．26
D．21
参考答案：C
解析：
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本数列为差后和数列，即1+(-1)=0，(-1)+0=(-1)，0+(-1)=(-1)，(-1)+(-1)=(-2)。即可推算出本数列的空缺项为26。故本题正确答案为C。
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