Which of the following integrals represents the length of the curve $\displaystyle \frac{x^2}{9}+\frac{y^2}{64}=1 ?$

A. $\displaystyle \int_{-8}^{\,8} \left(1+\frac{64 x^2}{9(9-x^2)}\right)^{1/2} \,dx$
B. $\displaystyle \int_{-3}^{\,3} \left(1+\frac{64 x^2}{9(3-x^2)}\right)^{1/2} \,dx$
C. $\displaystyle \int_{-3}^{\,3} \left(1+\frac{64 x^2}{8(3-x^2)}\right)^{1/2} \,dx$
D. $\displaystyle \int_{-8}^{\,8} \left(1+\frac{64 x^2}{9(3-x^2)}\right)^{1/2} \,dx$
E. $\displaystyle \int_{-3}^{\,3} \left(1+\frac{64 x^2}{9(9-x^2)}\right)^{1/2}\,dx$
F. $\displaystyle \int_{-8}^{\,8} \left(1+\frac{64 x^2}{8(3-x^2)}\right)^{1/2} \,dx$