[单选题]
A coin of mass $m$ rests on a turntable a distance $r$ from the axis of rotation. The turntable rotates with a frequency of $f$. What is the minimum coefficient of static friction between the turntable and the coin if the coin is not to slip?
A. $(4\pi^2 f^2 r)/g$
B. $(4\pi^2 f r^2)/g$
C. $(4\pi f^2 r)/g$
D. $(4\pi f r^2)/g$
