_________ it takes many years to gain the skills of an expert.

A．It is well known to us that

B．As is known to us that

C．As we know that

D．That is well known to us

治疗经行头痛血虚证，应首选的方剂是（）

A.八珍汤

B.十全大补汤

C.人参养荣汤

D.归脾汤

E.补中益气汤