Third
every action has an opposite and equal reaction
Answer:
C is halved
Explanation:
The frequency and the wavelength of a wave are related by the equation:

where
v is the speed of the wave
f is the frequency
is the wavelength
From the equation above, we see that for a given wave, if the wave is travelling in the same medium (and so, its speed is not changing), then the frequency and the wavelength are inversely proportional to each other.
Therefore, if the frequency doubles, the wavelength will halve in order to keep the speed constant:

Answer:
A: Sound waves produce an echo
Explanation:
because the cup is a hard smooth surface you can hear the voice