题目内容
19．-How long is it _____ you started the project of protecting the environment？（ ）| A． | before | B． | since | C． | if | D． | because |
分析 --自从你开始这个保护环境的项目以来多久了？
解答 答案：B．before意思是"在…以前"，用于结构"It+be+段时间+before…"中时，意思是"在…之后才…"；since意思是"自从…以来"，用于结构"It is+段时间+since…"或"It has been+段时间+since…"意思是"从…以来已经多长时间了"；if意思是"如果"；because意思是"因为"．How long 表示多长时间，主要用来对一段时间提问．根据句意"自从你开始这个保护环境的项目以来多久了？"所以要用since，故选B．
点评 首先要掌握这几个连词的意思以及用法，然后结合具体的题目，就可以作出正确选择．
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