$$\mu'_{(r)}$$ and $$\mu'_r$$ represent the factorial moment of order r about the origin and $$r^{th}$$ moment about the origin of the distribution $$x_i \mid f_i,i = 1,2,...n$$. The value of $$\mu'_2$$ equals to:

A

$$\mu'_{(1)}$$ $$^2$$

B

$$\mu'_{(2)} - \mu'_{(1)}$$

C

$$\mu'_{(2)} + \mu'_{(1)}$$

D

$$\mu'_{(2)}$$