If $$n_{1} = 10,n_{2} = 5$$ are the sizes of a male and female student group with mean ages $$\overline{x_{1}} = 10, \overline{x_{2}} = 4$$, respectively, with an equal standard deviation $$\sigma_{1} = 1, = \sigma_ 2$$ the standard deviation of the combined series with size $$n_{1} + n_{2} $$, and combined mean $$\overline{x} = 8$$ is equal to:

A

$$\sqrt{3}$$

B

9

C

135

D

3