Mark’s cost does not vary directly with time spent talking because A graph titled Phone Plan Costs shows Mark's cost and Nikiya's cost and shows Time Spent Talking (hours) on the x-axis and Cost (dollar sign) on the y-axis. A straight line showing Mark's cost starts at (0, 20) and goes through (3, 40). A straight line showing Nikiya's cost starts at (0, 0) and goes through (2, 30).