If a centrifugal governor operates between speed limits $\omega _{1}$ and $\omega _{2}$ 
, then what is its sensitivity equal to?
(a) $\frac{\omega _{1}+\omega _{2}}{ \omega _{1}-\omega _{2}}$
(b)$\frac{\omega _{1}+\omega _{2}}{2\left ( \omega _{2} -\omega _{1}\right )}$
(c)$\frac{\omega _{2}-\omega _{1}}{2\left ( \omega _{2} +\omega _{1}\right )}$
(d) $\frac{\omega _{2}+\omega _{1}}{ \omega _{2}+\omega _{1}}$