A heavy ball of mass $M$ is suspended from the ceiling of a car by a light string of mass $m$ ($m \ll M$). When the car is at rest, the speed of transverse waves in the string is $60 \mathrm {~ms} ^ { - 1 }$. When the car has acceleration $a$, the wavespeed increases to $60.5 \mathrm {~ms} ^ { - 1 }$. The value of $a$, in terms of gravitational acceleration $g$, is closest to\\
(1) $\frac { g } { 10 }$\\
(2) $\frac { g } { 20 }$\\
(3) $\frac { g } { 5 }$\\
(4) $\frac { g } { 30 }$