For each of the indefinite integrals below, select which of the following trig substitutions would be most helpful in evaluating the integral. Do not evaluate the integrals.

A. $x = 3 \tan \theta$
B. $x = 3 \sin \theta$
C. $x = 3 \sec \theta$

? A B C  1. $\displaystyle \int \frac{dx}{(9 - x^2)^{3/2}}$

? A B C  2. $\displaystyle \int x^2 \sqrt{9 + x^2} \, dx$

? A B C  3. $\displaystyle \int (x^2 - 9)^{5/2} \, dx$

? A B C  4. $\displaystyle \int \frac{x^2 \, dx}{\sqrt{9 - x^2}}$

? A B C  5. $\displaystyle \int \frac{dx}{(9 + x^2)^3}$