Some accountants calculate with simple adding machines, and some use complex computers. One can perform more calculations in less time with a computer than with an adding machine. Ttherefore, assuming the costs of using the two types of machines are equal, an accountant who uses a computer generally can earn more per hour than an accountant who uses an adding machine. Which one of the following is an assumption that would make the conclusion in the passage a logical one? ['The more hours an accountant spends on the job, the more money he or she will earn.', 'The more calculations an accountant performs, the more money he or she will earn.', 'In general, accountants vary in terms of the number of calculations they make and the amount of money they earn.', 'More accountants use computers than use adding machines.']