Last year, 1,000 cases of cough syrup were sold at $10; this year, 1,200 cases were sold at $12. The most probable interpretation of these data is that the:

a.

supply and demand curves are shifting to the right.

b.

supply and demand curves are shifting to the left.

c.

demand curve has shifted to the right, with no change in supply.

d.

supply curve has shifted to the left, with no change in demand.