问题
选择题
_______ entering the classroom, remember to take your seat and turn off your cell phone.
A．At
B．From
C．Upon
D．As
答案
答案：C
本题考查介词辨析。介词 on/upon后跟 V-ing 形式作“在……时；一……就……”讲，相当于 as soon as 引导的状语从句。
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