常用的超声波脉冲是（）
A、单纯的正弦波
B、有多种频率成分的正弦波叠加而成的机械波
C、方波
D、驻波
参考答案：B
简述前加速的优点及适用场合。
Halon extinguishers used on a class C fire should be directed at the（）.
A.base of the equipment
B.top of the equipment
C.power source
D.source of the fire
