单选题____AbeforeBunlessCuntilDbecause
单选题
____
A
before
B
unless
C
until
D
because
参考解析
解析：
before引导时间状语从句。unless引导条件状语从句。until引导时间状语从句。because引导原因状语从句，解释前面“engage”的原因。故D项为正确答案。
before引导时间状语从句。unless引导条件状语从句。until引导时间状语从句。because引导原因状语从句，解释前面“engage”的原因。故D项为正确答案。
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