用于保护瘘口周围皮肤的常用药膏是（）

A.硫黄软膏

B.氧化锌软膏

C.氟轻松软膏

D.红霉素软膏

E.磺胺软膏

He was riding to school         it began to rain.

A．as

B．so

C．while

D．when