Explanation:
Given that,
Bill is riding his bicycle at 5 m/s eastward: and Carlos is driving his car at 15 m/s westward.
Taking eastward as positive direction, we have:
is the velocity of Bill with respect to Amy (which is stationary)
is the velocity of Carlos with respect to Amy.
Bill is moving 5 m/s eastward compared to Amy at rest, so the velocity of Bill's reference frame is

Therefore, Carlos velocity in Bill's reference frame will be

So, the magnitude is 20 m/s and the direction is westward (negative sign).
The relationship between the frequency and wavelength of a wave is given by the equation:
v=λf, where v is the velocity of the wave, λ is the wavelength and f is the frequency.
If we divide the equation by f we get:
λ=v/f
From here we see that the wavelength and frequency are inversely proportional. So as the frequency increases the wavelength decreases.
So the second statement is true: As the frequency of a wave increases, the shorter the wavelength is.
Since the new distance is 3 times the old distance,
the new force is (1/3²) = 1/9th of the old force.
That's kind-of Choice-D, but I really don't like the way choice-D is worded.
"9 times smaller" is really pretty meaningless.