He _____ football regularly for many years when he was young.

A．was playing

B．played

C．has played

D．had played

测定巨噬细胞的吞噬率和吞噬指数，常采用的方法是（）

A.NBT还原试验

B.杀菌试验

C.斑蟊发泡法

D.趋化试验

E.白色念珠菌吞噬试验