决定湿温病湿热之邪从化的主要因素是（）

A.宗气的盛衰

B.病程的长短

C.肾气的盛衰

D.中气的盛衰

E.卫气的盛衰

After the class was _____, some of the students were asked to remain.[ ]

A. at an end      

B. in the end      

C. by the end      

D. at the end