A heavy ball of mass M is suspended from the ceiling of a car by a light string of mass m (m≪M). When the car is at rest, the speed of transverse waves in the string is 60 ms−1. When the car has acceleration a, the wave-speed increases to 60.5 ms−1. The value of a, in terms of gravitational acceleration g, is closest to:
(1) 30g
(2) 5g
(3) 10g
(4) 20g