The profitability of a business is reduced by anything that undermines employee morale. This is why paying senior staff with stock options, which allows them to earn more when the enterprise prospers, is not a wise policy because it increases dramatically the difference in income between senior staff and employees who are paid only a fixed salary. Which one of the following is an assumption on which the argument depends? ['Large income differences between fixed-salary employees and senior staff tend to undermine employee morale.', 'Business firms that pay senior staff with stock options are less profitable than other firms.', 'Employees whose incomes rise as the profits of their employers rise are more productive than those paid only a fixed salary.', 'Reductions in the profitability of a company are usually due to low employee morale.']