Indicate whether each of the following statements is true, false, or uncertain, and justify your answer.

a. If everyone has the same marginal rate of substitution, then the allocation of resources is Pareto efficient.

b. If the allocation of resources is Pareto efficient, then everyone has the same marginal rate of substitution.

c. A policy change increases social welfare if, and only if, it represents a Pareto improvement.

d. A reallocation from a point within the utility possibilities curve to a point on the utility possibilities curve results in a Pareto improvement.