6., RST Company offers a qualified retirement plan. Each employee contributes 4 percent of his or her pretax income to the plan, and RST matches each employee's contribution. An employee's benefit at retirement is determined by his or her account balance at the time of retirement. What type of retirement plan does RST offer? a. Defined contribution plan. b. Defined benefit, flat percentage of annual earnings c. Defined benefit, unit-credit formula d. Defined benefit, flat dollar amount for all employees