A Firm Pays Pam $40 Per Hour to Assemble Personal

A firm pays Pam $40 per hour to assemble personal computers. Each day, Pam can assemble 4 computers if she works 1 hour, 7 computers if she works 2 hours, 9 computers if she works 3 hours, and 10 computers if she works 4 hours. Pam cannot work more than 4 hours day. Each computer consists of a motherboard, a hard drive, a case, a monitor, a keyboard, and a mouse. The total cost of these parts is $600 per computer. If Jane works for 6 hours she can rent out 9 apartments, and if she works for 7 hours she can rent out 12 apartments. The marginal benefit of Jane's 7^th hour of work equals:

A) 12 apartments
B) 9 apartments
C) 3 apartments
D) 1 apartment