To an economist, scarcity means that: a. resources are unlimited but people's desires are limited. b. a good is unavailable even at very high prices. c. at a zero price, the available quantity of a good is insufficient to meet people's wants. d. at the current market price, the amount available is less than the amount that people want and are willing to pay for. e. it is very time-consuming to find a good.

Because of the violent nature of the sport, professional american football players are at risk of suffering concussions (or brain injuries) during the violent collisions that occur between players during the game. violent collisions involving hits to the head are particularly risky. suppose the commissioner of the national football league (nfl) institutes a rule requiring players to wear a new, lighter helmet that is designed to reduce head trauma during collisions between players on the field. while the new helmets the probability of a concussion resulting from each individual collision, at the same time the new helmets could also give football players an incentive to play more , which could potentially the amount of violent collisions and thus the number of concussions suffered by nfl players, all else equal.