Answer:
D. impossible to estimate
Explanation:
According to the doppler effect, the frequency of the wave will increase.
let u be the speed of the submarine,
let the speed of sound in water be c,
let initial frequency of the sent out wave be f,
let frequency of received wave be f'
The distance occupied by the new wave will be (c - u), therefore, the new wavelenght will be (c - u)/f
Frequency = speed of sound ÷ wavelenght
therefore the new Freq f' will be
f' = cf/(c - u) which will be shorter than the initial wavelenght.
But we do not know the speed of the submarine, so we can't estimate.
