A mass-spring-damper system with force as input and displacement of the mass as output has a transfer function $G(s)=1/(s^2+24s+900)$. A force input $F(t)=10\sin(70t)$ newtons is applied at time $t=0\;s$. A beam from an optical stroboscope is focused on the mass. In steady state, the strobe frequency in hertz at which the mass appears to be stationary is
- $5/\pi$
- $15/\pi$
- $35/\pi$
- $50/\pi$