To create an expression for income, a company multiplies the number of items they sell by the price of each item. If the income can be represented by the expression -0.5x2+8x, and the price per item is represented by x, which of the following expressions will represent the number of items sold.
18x
1: 8x
2:-0.5x2 + 8

3:-0.5x2 + 8x

4:-0.5x+8