For a continuous rate function r on [a,b], which of the following conditions is sufficient to guarantee that
∫abr(x)dx=∫ab∣r(x)∣dx?
a) r(x)≤0 for all x in [a,b]
b) ∫abr(x)dx=0
c) r(x)≥0 for all x in [a,b]
d) r(x) attains both positive and negative values on [a,b]