Sales of a new model of compact disc player are approximated by the function  S(x) = 1000 - 800e^-x, where S(x) is in appropriate units and x represents the number of years the disc player has been on the market.
(a) Find the sales during year 0.
(b) In how many years will sales reach 500 units?
(c) Will sales ever reach 1000 units?
(d) Is there a limit on sales for this product? If so, what is it?