The integrating factor of equation $$\sec^2 y \frac{dy}{dx} + x \tan y = x^3$$ is

A

$$[e]^{\frac{x^2}{2}}$$

B

$$[e]^{\frac{-x^2}{2}}$$

C

$$[e]^{\frac{x}{2}}$$

D

$$[e]^{\frac{-x}{2}}$$