A. The Demand Curve
Explanation

A demand curve shows the relationship between the price of a good and the quantity consumers are willing to purchase, holding all other factors constant. The law of demand states that as the price rises, the quantity demanded falls, and vice versa, giving the curve its downward slope.

Each point represents a price and quantity pair from the demand schedule. As the price of gasoline falls from $2.20 to $1.00 per gallon, the quantity demanded nearly doubles from 420 to 800 million gallons. Lower prices reduce the opportunity cost of purchasing gasoline, so consumers buy more.

The demand shift slider explores what happens when non-price factors change (income, preferences, substitute prices). Moving along the curve is a change in quantity demanded (price-driven); shifting the entire curve is a change in demand (driven by other factors).

Elasticity
Demand Schedule
Price ($/gal)Qty (M gal)ΔPΔQ
Shift Demand
H-shift = change in quantity demanded at every price level. V-shift = change in willingness to pay for every quantity.
Selected Point
Key
Demand curve D
Original D (when shifted)
Price and quantity points
\( Q_d = f(P) \quad \text{with} \quad \frac{\Delta Q_d}{\Delta P} < 0 \)