An engine whistling at a constant frequency n0 and moving with a constant velocity goes past a stationary observer. As the engine crosses him, the frequency of the sound heard by him changes by a factor f. The actual differences in the frequencies of the sound heard by him before and after the engine crosses him is
(a) 1/2n0(1 - f2)
(b) 1/2n0((1 - f2)/f)
(c) n0(1 - f/1 + f)
(d) 1/2n0(1 - f/1 + f)