The number of sales per month, S, is a function of the amount, a (in dollars), spent on advertising that month, so S = f(a). 

(a) Interpret the statement f(1000) = 3500. 

(b) Which of the graphs in Figure 1.14 is more likely to represent this function? 

(c) What does the vertical intercept of the graph of this function represent, in terms of sales and advertising?

Figure 1.14

(1) S a (II) S D