Use substitution to find the indefinite integral 6x + 1 (3x2 + x) 4 dx . . . Describe the most appropriate substitution case and the values of u and du. Select the correct choice below and fill in the answer boxes within your choice O A. Substitute u for the quantity raised to an integer power. Let u = so that du = () dx O B. Substitute u for the entire quantity in the denominator. Let u = so that du = ( ) dx O C. Substitute u for the quantity in the numerator Let u = , so that du = () dx Use the substitution to evaluate the integral 6x + 1 (3x2 + x) 4 a dx =