The demand curve is negatively sloped because as the price of a good rises, the quantity demanded increases. Price and quantity demanded are negatively related due to the law of diminishing marginal utility. To entice a consumer to consume more, the price must fall because the marginal utility derived from consumption of another unit is falling. In your own words, provide one sentence definitions of ceteris paribus, marginal analysis, and opportunity cost. Ceteris paribus is Latin for all else being equal and means all other relevant things remaining the same.
Examples are marginal utility, marginal cost, marginal product, etc.
The opportunity cost of an action is the value of the best alternative, what you would have done instead. In a world of scarcity, everything has an opportunity cost. Assume that the current price is below market equilibrium, draw and describe how the market adjusts to equilibrium. At a price below the equilibrium price, the quantity supplied will be less than the quantity demanded. A shortage results and firms respond by raising their price.