—The duty of Project Hope is to help poor children, isn't it?      

—Yes, it has built many schools _____ those children can study happily.       [ ]

A. where                      

B. when        

C. which                      

D. what

下列对一般牙拔除时的描述不正确的是（）

A.指用钳、挺就能拔除的手术

B.拔牙前必须分离牙龈

C.拔除牙齿时需要同时使用摇动、扭转、牵引三种力

D.拔牙后应检查拔除的牙齿是否完整

E.拔牙2小时后方可进食