the two are only several days apart in age, they seem to belong to wholly different generations.
A. While
B. When
C. As
D. Since
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(答案→)A. While
B. When
C. As
D. Since
解析：考查连词：A. While虽然，然而，当…时候，B. When 当..时候C. As因为，当…时候，正如，D. Since自从，既然，句意：两个人相距只有几天的年龄，但他们似乎属于完全不同的世代。while表示虽然，有转折的意思。选A。