This type of bamboo grows very fast and it will take only three to five years it can be used in various fields .
A. before
B. since
C. when
D. as
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(答案→)A. before
B. since
C. when
D. as
解析：：考查固定句型。It be 一段时间 before从句。意为：一段时间以后某事才发生。句意：这种竹子长得很快。三到五年以后就可以在不同的领域被使用了。巩固A正确。