What is the completely factored form of $x^4+8 x^2-9$?
A$(x+1)(x-1)\left(x^2+9\right)$
B$(x+1)(x-1)(x+3)(x+3)$
C$(x+1)(x+1)(x+3)(x+3)$
D$\left(x^2-1\right)(x+3)(x-3)$
A$(x+1)(x-1)\left(x^2+9\right)$
B$(x+1)(x-1)(x+3)(x+3)$
C$(x+1)(x+1)(x+3)(x+3)$
D$\left(x^2-1\right)(x+3)(x-3)$