For any $$\theta\in\left(\frac{\pi}{4},\frac{\pi}{2}\right)$$, the expression $$3\left(\sin\theta\ -\cos\theta\ \right)^4+6\left(\sin\theta\ +\cos\theta\ \right)^2+4\sin^6\theta\ $$ equals:

A

$$13 - 4\cos^2\theta + 6\cos^4\theta$$

B

$$13 - 4\cos^2\theta + 6\sin^2\theta\cos^2\theta$$

C

$$13 - 4\cos^6\theta$$

D

$$13 - 4\cos^4\theta + 2\sin^2\theta\cos^2\theta$$