The value of $$\left(a^{\frac{2}{3}} + 2a^{\frac{1}{2}} + 3a^{\frac{1}{3}} + 2a^{\frac{1}{6}} + 1\right)\left(a^{\frac{1}{3}} - 2a^{\frac{1}{6}} + 1\right)-a^{\frac{1}{2}}\left(a^{\frac{1}{2}} - 2\right)$$, when a = 7, is:

A

0

B

7

C

1

D

$$\sqrt{7}$$