Volumes Using Cross-Sections and Cylindrical Shells - Lecture 17 (Calculus 1)

What is the central mental shift the lecture asks for when moving from area problems to volume problems?

1. A.Memorizing closed-form formulas for every standard solid you might encounter
2. B.Viewing an integral as an infinite sum of arbitrarily small contributions, not just an area
3. C.Switching from Riemann sums to limits of secant approximations
4. D.Replacing dx with dV in every integrand to indicate three dimensions