Which of the following substitutions is the most appropriate to evaluate ∫x3x2+5dx\int x^3 \sqrt{x^2 + 5} dx∫x3x2+5dx?
u=x3u = x^3u=x3
u=x2+5u = \sqrt{x^2 + 5}u=x2+5
u=x2+5u = x^2 + 5u=x2+5
u=x+5u = x + 5u=x+5