It seems that you have missed the necessary options in order for us to answer this question so I had to look for it. Anyway, here is the answer. Suppose smith wants one ipod no matter what the price is between $0 and $150, jones wants one ipod no matter what the price is between $0 and $200, and young wants one ipod no matter what the price is between $0 and $250. In this case, each individual buyer's demand curve will be VERTICAL and the market demand curve will be DOWNWARD SLOPING. Hope this helps.