Which of the following is true?

a. The law of supply states that the higher (lower) the price of a good, the greater (smaller) the quantity supplied.

b. The relationship between price and quantity supplied is positive because profit opportunities are greater at higher prices and because the higher production costs of increased output mean that suppliers will require higher prices.

c. The market supply curve is a graphical representation of the number of goods and services that suppliers are willing and able to supply at various prices.

d. All of these are true.