Demand Curves Have a Negative Slope Because

Demand curves have a negative slope because

A) firms tend to produce less of a good that is more costly to produce.
B) the substitution effect always leads consumers to substitute higher quality goods for lower quality goods.
C) the substitution effect always causes consumers to try to substitute away from the consumption of a commodity when the commodity's price rises.
D) an increase in price reduces real income and the income effect always causes consumers to reduce consumption of a commodity when income falls.