Please ignore the chosen answer, it was clicked by accident but please help :(

Which of the following contains a variable?

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

Will give brainliestbased on the topographic map of mt. st. helens, what is the contour interval if the volcano height is 2,950 m?