Toss Up - Math - Multiple Choice

The total distance traveled, given by the equation `f(t_1)Deltat + f(t_2)Deltat + ... f(t_n)Deltat` , (read as: f of t sub 1, delta t, + f of t sub 2, delta t, + all values up to f of t sub n, delta t), for which f is an increasing function, can be used to find which of the following between the values `t = a` and `t = b`:
W) right-hand sum
X) left-hand sum
Y) upper bound
Z) endless bound