Set up the integral representing the volume of the region bounded by the ellipse ax² + by² = r² about the x-axis. Which of the following options correctly represents the integral?

A) ∫[0, r] π(ax² + by²) dx
B) ∫[0, r] π(ax² - by²) dx
C) ∫[0, r] π(ax² + by²) dy
D) ∫[0, r] π(ax² - by²) dy