15. Which of the following is true?

a.

The market demand curve for a product is the vertical summation of the demand curves of the individuals in the market.

b.

A fall in a good's price leads to a decrease in quantity demanded, illustrated by moving along a demand curve.

c.

According to the law of demand, other things equal, when the price of a good or service falls, demand increases.

d.

A change in demand for chocolate bars is caused by a change in the price of chocolate bars.

e.

None of the above is true.