The coefficient of $$x^{-7}$$ in the binomial expansion of $$\left(3x^{\frac{1}{5}} + \frac{4}{\sqrt{x}}\right)^{21}$$

A

$$21_{C_{5}}3^{16}4^{5}$$

B

$$21_{C_{5}}3^{5}4^{16}$$

C

$$21_{C_{6}}3^{15}4^{6}$$

D

$$21_{C_{6}}3^{6}4^{15}$$