List x1, x2, x3, x4 where xi is the left endpoint of the four equal intervals used to estimate the area under the curve of f(x) between x = 4 and x = 6.

A 4, 4.5, 5, 5.5
B 4.5, 5, 5.5, 6
C 4.25, 4.75, 5.25, 5.75
D 4, 4.2, 5.4, 6