防止彩色信号倒错（混迭）的方法是
A．用高频超声
B．下移零位基线
C．低通滤波
D．高增益
E．低速标尺
参考答案：B
They had just taken their seats, then _____. [ ]
A. the chairman came
B. the chairman comes
C. came the chairman
D. comes the chairman
易引起大出血的消化性溃疡为
A．球后溃疡B．幽门管溃疡C．胃小弯溃疡D．十二指肠球部溃疡E．复合性溃疡
