食管的生理狭窄有（）
A.环状软骨下缘
B.主动脉弓及气管分叉后方
C.左心房压迫食管前壁
D.膈肌食管裂孔处
E.食管-胃交界处
参考答案：A, B, D
方程x2-mx+n=0的两个根，一个是-1，另一个是1，则m=（ ），n=（ ）。
The factory was built in a secret place, around _________ high mountains.
A．which was
B．it was
C．which were
D．them were
