造成混响伪像的主要因素是:()
A.电噪声
B.TGC设置不当
C.入射角过小
D.存在两个或多个强反射界面
E.声束剖面过大
参考答案:D
The plane _____ at 7:00 p.m., so I have to be at the airport by 6:40 at the latest.
A.has left
B.is to leave
C.will have left
D.leaves
简述腰下三角的境界及临床意义。