突然停煤气或煤气降压可能造成的后果是（），回火发生爆炸。
参考答案：混合气体进入煤气管道
Nearly all the streets are in straight lines, _____ from east to west. Those _____ run from north to south are
called avenues. [ ]
A. running; that
B. run; who
C. running; who
D. run; that
增强超声造影效果应用哪项技术()
A．连续波多普勒
B．频谱多普勒技术
C．谐波成像
D．二维超声显像
E．M型超声
