The sales of a company are related to its expenditures on research by S(x) = 1000 + 60√x + 12x, where S(x) gives sales in millions when x thousand dollars is spent on research. Find and interpret dS/dx if the following amounts are spent on research.
(a) $9000
(b) $16,000
(c) $25,000
(d) As the amount spent on research increases, what happens to sales?