超声扫查时大血管内血液流速可以用多普勒效应测定。
参考答案：对
He_______ in this city since his family moved to Nanchong.
A. live B. lived C. has lived
当环路的相位裕度接近于0°，在滤波器中增加（）来改善环路的稳定性。
A、一个零点
B、一个基点
C、两个零点
D、两个极点
