An increase in demand is defined as an increase in the quantity that people are willing and able to purchase at

A) at least one price.
B) some prices.
C) all prices.

An increase in demand means that at all prices, the quantity that consumers are willing and able to purchase has increased. This is different from an increase in quantity demanded, which occurs at a specific price due to a movement along the demand curve. An increase in demand shifts the entire demand curve to the right.