The concept of demand is best described as:
a. the total satisfaction that consuming a good provides people at different prices.

b. the quantity of a good or a service that people will offer for sale at different possible prices.

c. the additional satisfaction derived from a quantity of goods and services obtained when income increases.

d. the quantity of a good or a service that people are willing and able to purchase at different possible prices.

e. the quantity of a good or service that consumers will substitute when the price of a good changes.