The graph represents the data cost for monthly Internet service for a cell phone.

On a coordinate plane, a graph titled Cell Phone Internet Service shows Gigabytes Used on the x-axis and Monthly Cost in dollars on the y-axis. A piecewise function has 3 lines. The 1st line has a closed circle at (0, 15), continues horizontally at y = 15, then has a closed circle at (2, 15). The 2nd line has an open circle at (2, 20) and then continues up to a closed circle at (6, 40). The 3rd line has an open circle at (6, 50) and then continues horizontally at y = 50.
Which function, C(x), represents the monthly cost in dollars in terms of x, the number of gigabytes used in a month?

C(x) = StartLayout enlarged left-brace 1st Row 1st column 15, 2nd column 0 less-than-or-equal-to x less-than-or-equal-to 2 2nd Row 1st column 5 x + 10, 2nd column 2 less-than x less-than-or-equal-to 6 3rd Row 1st column 50, 2nd column 6 less-than x EndLayout
C(x) = StartLayout enlarged left-brace 1st Row 1st column 15, 2nd column 0 less-than-or-equal-to x less-than 2 2nd Row 1st column 5 x + 20, 2nd column 2 less-than-or-equal-to x less-than 6 3rd Row 1st column 50, 2nd column 6 less-than-or-equal-to x EndLayout
C(x) = StartLayout enlarged left-brace 1st Row 1st column 15x, 2nd column 0 less-than-or-equal-to x less-than-or-equal-to 2 2nd Row 1st column 20 x + 5, 2nd column 2 less-than x less-than-or-equal-to 6 3rd Row 1st column 50 x, 2nd column 6 less-than x EndLayout
C(x) = StartLayout enlarged left-brace 1st Row 1st column 15x, 2nd column 0 less-than-or-equal-to x less-than 2 2nd Row 1st column 5 x + 10, 2nd column 2 less-than-or-equal-to x less-than 6 3rd Row 1st column 50 x, 2nd column 6 less-than-or-equal-to x EndLayout