$$\frac{2}{\sqrt{10 + 2\sqrt{21}}} - \frac{1}{\sqrt{12 - 2\sqrt{35}}} + \frac{1}{\sqrt{8 - 2\sqrt{15}}} =$$

A

$$\sqrt{7}$$

B

$$\sqrt{5}$$

C

$$\sqrt{3}$$

D

$$0$$