QuestionConsider $\int 6 x^{7}\left(4 x^{8}+9\right)^{6} d x$ What is the most appropriate u-substitution? Select only ONE answer. $u=\left(4 x^{8}+9\right)^{6}$ $u=6 x^{7}$ $u=4 x^{8}+9$ $u=x$