单选题
Last year the employees in our department were so busy ______ they were not able to take a vacation.
A、
which
B、
what
C、
who
D、
that
【正确答案】
D
【答案解析】
固定搭配
so+形容词+that+从句，为固定搭配，意为“如此…以至于”，故选D。
so只能与that搭配，与其他三个选项无法搭配。
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