entering the classroom remember to take your seat and turn off your cell phone.A. AtB. F

问题补充：

entering the classroom, remember to take your seat and turn off your cell phone.

A. At

B. From

C. Upon

D. As

答案：

A. At

B. From

(答案→)C. Upon

D. As

解析：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。