1) Sound is the kind of  a) work b) energy c) force d) none 2) vibration is a) visible b) invisible  c) can both (a) and (b)  d) non 3) sound can not be produce without vibrations a) true b) false

class 9th

由
打印
嵌入

排行榜

视觉风格

选项

切换模板

恢复自动保存：？