A source, approaching with speed 𝑢 towards the open end of a stationary pipe of length 𝐿, is emitting a sound of frequency $$f_s$$. The farther end of the pipe is closed. The speed of sound in air is v and $$f_0$$ is the fundamental frequency of the pipe. For which of the following combination(s) of u and $$f_s$$, will the sound reaching the pipe lead to a resonance?

A

u = 0.8v and $$f_s = f_0$$

B

u = 0.8v and $$f_s = 2 f_0$$

C

u = 0.8v and $$f_s = 0.5 f_0$$

D

u = 0.8v and $$f_s = 1.5 f_0$$