Answer: vacuum
Explanation: The vacuum is the medium from where the sound wave cannot pass. A vacuum is basically an area without any air. Since the sound wave is a mechanical wave that's why it cannot travel through a medium where there is no matter of vibrations to works in, i.e, it can't travel through a vacuum.
Answer:
It cancels recoil.
Explanation:
For each action there is an equal an opposite reaction.
The principle of conservation of momentum tell us that if a single spore were ejected the fern would suffer a recoil from it. This recoil would take energy and speed from the spore. But if they are ejected in pairs the recoil is canceled and all the energy is transferred to the spores resulting in higher speeds.
Answer:
0.002833 sec
Explanation:
Speed of light in vacuum is 
Given distance = 850 km = 850×1000=850000 m
We have to calculate the time that light take to travel the distance 850 km
Time 
So the time taken by light to travel 850 km is 0.002833 sec
B. Because they are used to see through objects
