The school year is divided into two semesters, is September through December.
A. the first of which
B. the first of them
C. the first of it
D. the first of that
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(答案→)A. the first of which
B. the first of them
C. the first of it
D. the first of that
解析：考查定语从句。 is September through December.是非限制性定语从句，先行词是semesters，them和it 不能引导定语从句，that不能引导非限制性定语从句，所以选A。