If $$n_1 = 10$$ and $$n_2 = 5$$, are the sizes, $$\overline{x}_1, = 7$$ and $$\overline{x}_2, = 4$$ are the means and $$\sigma_1 = 1$$ and $$\sigma_2 = 1$$ are the standard deviations of two series of data. If combined mean $$\overline{x}_2, = 6$$, then the variance of the combined series with size $$n_1 + n_2$$ is equal to:

A

3

B

1

C

2

D

9