The ratio of the height of a porter governor (when the length of arms and links are equal) to the height of watt governor is: (m: mass of the ball; M : mass of the sleeve)

A

$$\frac{m}{m + M}$$

B

$$\frac{M}{m + M}$$

C

$$\frac{m + M}{m}$$

D

$$\frac{m + M}{M}$$