Sales of CDs have greatly increased since the early 1990s, when people_____to enjoy the advantages of this new technology.

A．begin

B．began

C．have begun

D．had begun

乳癌根治术后做胸壁照射时，常用的照射技术为（）。

A.高能X线垂直对穿

B.电子束切线照射

C.电子束照射

D.深部X线垂直照射

E.深部X线切线照射