Water flows into and out of a reservoir at a rate r(t) (in liters per minute) for t>0. The volume of water in the reservoir at time t is R(t)=∫0tr(s)ds. Suppose r(t)>0 for 0<t<6, r(t)<0 for 6<t<9, and r(t)>0 for t>9. Which of the following best describes the behavior of R?
a) R increases on (0,6), increases on (6,9), and increases on (9,∞).
b) R increases on (0,6), increases on (6,9), and decreases on (9,∞).
c) R increases on (0,6), decreases on (6,9), and increases on (9,∞).
d) R increases on (0,6), decreases on (6,9), and decreases on (9,∞).