If $$\sqrt{15 - 2\sqrt{35} - 2\sqrt{15} + 2\sqrt{21}} = \sqrt{a} - \sqrt{b} + \sqrt{c}$$ then $$a^2 - c^2 =$$

A

$$b^2 - 1$$

B

$$b^2 + 1$$

C

b(b + 3)

D

b(b + 1)