Nowadays, some students would not like to live in a dormitory. Instead, they rented houses outside the school. There are advantages for them all. Living in a dormitory can increase friendship and cooperation among classmates, but of course it is safer．Besides, a dormitory is more like a society, where enables students to learn to get along with the each other，thus preparing the social life later. As for renting houses to live lonely, it’s better for your study as you can feel freely in your own room without being interrupted by others. As a result，you can have a plenty of time to devote yourself to review your lessons wholeheartedly. But there may appear a few safety problem as you live outside the school. Anyhow as a student, I prefer living in a dormitory.

补阳还五汤中重用黄芪的意义是（）。

A．.益气升阳

B．大补肿肺之气，以裕生血之源

C．益气固表

D．大补脾胃元气，使气旺以促血行

E．益气托毒外出