Tom ______ up early. But now he doesn't get up early. [ ]

A.used to get

B.is used get

C.was used to get

D.used getting

下列何种方法可用于获得纯蛋白质（）

A.琼脂糖凝胶电泳

B.离子交换层析

C.免疫固定电泳

D.亲和层析

E.透析