4. Rafael has a business selling computers. He buys computers from the manufacturer for $450 each and sells them for $800. Each month, he must also pay fixed costs of $3,000 for rent and utilities for his store. If he sells n computers in a month, which of the following equations can be used to find his profit? a. P = n($800 - $450) b. P = n($800-$450 $3,000) c. P = $3,000 x n($800 - $450) d. P = n($800 - $450) - $3,000