For your business, you buy a certain type of item from the manufacturer, and then you sell it to your customers at a mark-up. That is, you sell it for more than you bought it. You are concerned with the following functions regarding this item.

• In month $t\text{,}$ the function $c(t)$ represents your cost to buy one of the items.
• In month $t\text{,}$ the function $p(t)$ represents the price you sell the item for.
• In month $t\text{,}$ the function $q(t)$ represents the total quantity of the items you sell that month.

Which function combination represents the total amount your customers paid you for all the items you sell? Answer: ? c(t)p(t) p(t)q(t) c(t)q(t)