A man is walking at speed vm in front of an ambulance travelling towards him at speed va. The siren on the ambulance is emitting a sound at speed c. Since the ambulance is moving faster than the man (i.e. vm<va), it overtakes the man later. What is the ratio of wavelength of the sound perceived by the man when the ambulance is approaching him to that when it is leaving him?
a) va−vmva+vm
b) c−vac+va
c) c+vac−va
d) c−vm+vac+vm−va
e) c+vm−vac−vm+va