21. The slope of a demand curve is almost always a. positive, because when people buy more of a good the cost of producing it will rise. b. positive, because the more money a person has, the more of a particular good will be bought. c. negative, because when people buy more of a good the cost of producing it will fall. d. negative, because with everything else equal, the same people will buy more of a good when its price is lower. e. positive, because as the price rises, people want to sell more of the good