The minimum and minimum speeds of a flywheel during a cycle are $$N_{1}  and  N_{2}$$ r.p.m. respectively. The coefficient of steadiness of the flywheel is

A

$$\frac{(N_{1}-N_{2})}{(N_{1}+N_{2})}$$

B

$$\frac{(N_{1}+N_{2})}{2(N_{1}-N_{2})}$$

C

$$\frac{2(N_{1}+N_{2})}{(N_{1}-N_{2})}$$

D

$$\frac{(N_{1}+N_{2})}{(N_{1}-N_{2})}$$