During the class Tom slipped out of the classroom the back door so as not to be found by

问题补充：

During the class, Tom slipped out of the classroom the back door so as not to be found by the teacher.

A. across

B. in

C. by

D. beside

答案：

A. across

B. in

(答案→)C. by

D. beside

解析：介词辨析。By表示方法和手段，意为：通过；结合句意可知C正确。句意：在上课期间，Tom从后面溜出教室，以免被发现。