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

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：句意：一进入教室，记得坐下关掉手机。Upon+名词或动名词，表示“一…”，at“在…地方，听见…”，from“来自”，As“做为”，所以选C。