(c) the loudness decreases
   (d) the sound suddenly stops
Ans: (a)
216. We see the flash of lightning before hearing the thunder because
   (a) light travels very much faster than sound
   (b) sound travels faster than light
   (c) lightning occurs nearer than thunder
   (d) both travel at the same speed
Ans: (a)
217. Sound waves
   (a) are electromagnetic waves
   (b) can easily pass through vacuum
   (c) are always transverse waves
   (d) require a material medium for propagation
Ans: (a)
218. While crossing a suspension bridge, the soldiers are ordered to break
   steps because
   (a) the marching in steps will involve a big strain on them
   (b) the resonant vibrations caused by their marching can damage the bridge
       increasing the amplitude to dangerous proportions
   (c) the marching may obstruct other traffic
   (d) rumblings of their marching may disturb the surroundings
Ans: (b)
219. Hydrophone is an instrument used
   (a) for measuring relative density of liquids
   (b) by ships to pass on messages
   (c) for determining the depth of ocean bottoms etc.
   (d) by deaf people to aid hearing
Ans: (c)
220. Bats flying in the dark avoid obstacles because
   (a) they can see very dearly only in darkness
   (b) they produce ultrasonic waves which guide them