A car is moving at a velocity of 17 ms^-1 towards an approaching bus that blows a horn at a frequency of 640 Hz on a straight track. The frequency of this horn appears to be 680 Hz to the car driver. If the velocity of sound in air is 340 ms^-1 , then the velocity of the approaching bus is:
A. 2 ms^-1
B. 4 ms^-1
C. 8 ms^-1
D. 10 ms^-1