A source, approaching with speed u towards the open end of a stationary pipe of length L, is emitting a sound of frequency f. The farther end of the pipe is closed. The speed of sound in air is v and fo is the fundamental frequency of the pipe. For which of the following combination(s) of u and fs, will the sound reaching the pipe lead to a resonance? (A) u=0.8v and fs = fo (B) u=0.8v and fs = 2fo (C) u = 0.8v and f = 0.5fo (D) u=0.5v and f = 1.5fo