Given contours of f(x, y) on a rectangle R with 0 ≤ x ≤ 30 and 0 ≤ y ≤ 15, using δx = 10 and δy = 5, find an overestimate and an underestimate for ∫f(x, y) da.

a) Overestimate, Underestimate
b) Underestimate, Overestimate
c) Overestimate, Overestimate
d) Underestimate, Underestimate