A good teacher must be able to hold his students’ attention ____ he reaches the end of the class.

A．when

B．after

C．until

D．unless

怀疑上消化道出血首选的检查为（）。

A.X线胃肠钡餐透视

B.X线Ba灌肠透视检查

C.便潜血检查

D.B型超声检查

E.胃镜检查