In a triangle $$ABC$$, if $$\cos A + 2\cos B + \cos C = 2$$ and the lengths of the sides opposite to the angles $$A$$ and $$C$$ are 3 and 7 respectively, then $$\cos A - \cos C$$ is equal to

A

$$\frac{9}{7}$$

B

$$\frac{10}{7}$$

C

$$\frac{5}{7}$$

D

$$\frac{3}{7}$$