答案：A

解析：

Drawing the vertical line "Number of Items = 10" and the horizontal line "Total Cost = "$20" divides the scatterplot into four regions, as shown in the figure below.

The lower right region lies below the line "Total Cost = $20" and to the right of the line "Number of Items = 10." Thus the points in this region are exactly the points that represent a purchase of more than 10 items at a total cost of less than $20. (Note that the points on the line "Number of Items = 10" represent a purchase of exactly 10 items, not more than 10 items and, thus, do not meet the given conditions.) There are four points in the lower right region. Therefore, four of the customers bought more than 10 items and spent less than $20.