问题补充：
(·贵州模拟)Tom, clean up the broken glasssomeone walks on it.
A. since
B. when
C. before
D. as
答案：
A. since
B. when
C. before
D. as
解析：选C。考查状语从句。句意: 汤姆, 把碎玻璃收拾干净, 以防有人踩上。此处before意为“以防; 免得”, 引导时间状语从句。
【变式训练】
①(·重庆模拟)—Hello, can I speak to Mr. Smith?
—I’m sorry, but it is 3 weekshe worked here.
A. before B. since C. after D. while
选B。考查状语从句。句意: ——喂, 我可以跟史密斯先生通电话吗? ——很抱歉, 他不在这里工作已经有三周了。此处考查句型: It is +一段时间+since从句(使用一般过去时)。此处since从句使用的是延续性动词, 表示该动作停止到现在多久了。
②It was nearly an hourthe sleeping pilleffect.
A. when; took B. before; took
C. since; takes D. before; would take
选B。考查状语从句和时态。句意: 过了几乎一小时那片安眠药才起作用。此处before引导一个时间状语从句, 句型为: It was some time before. . . did. . . 表示“过了多久才……”。
