The total number of sales for all four months is 5 (Jan) + 7 (Feb) + 4 (Mar) + 8 (Apr) = 24.
The average number of sales per month is 24 (total sales) ÷ 4 (number of months) = 6.
Each dollar sign represents an average of 6 sales.
Therefore, the correct answer is:
C) 20