For purposes of assessing income taxes, there are three official income levels for workers in a small country: high, medium, and low. For the last hour on the job during a 40-hour workweek, a high-income worker pays a marginal income tax rate of 15 percent, a medium-income worker pays a marginal tax rate of 20 percent, and a low-income worker is assessed a 25 percent marginal income tax rate. Based only on this information, does this nation's income tax system appear to be progressive, proportional, or regressive?