Question7. $\frac{d}{d x} \cos ^{2}\left(x^{3}\right)=$ (A) $6 x^{2} \sin \left(x^{3}\right) \cos \left(x^{3}\right)$ (B) $6 x^{2} \cos \left(x^{3}\right)$ (C) $\sin ^{2}\left(x^{3}\right)$ (D) $-6 x^{2} \sin \left(x^{3}\right) \cos \left(x^{3}\right)$ (E) $-2 \sin \left(x^{3}\right) \cos \left(x^{3}\right)$