【解题分析】第一空格后的介词短语为in their geographical distribution，故空格中所填的词为某种分布特征，进一步可知冒号后应从分布特征的角度去阅读。冒号前后表达的分布特征相同的是答案a。

　　【重点词条】uniformity 1: the quality or state of being uniform 2: an instance of uniformity

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