Marginal external cost equals marginal private cost minus marginal social cost

The Coase theorem asserts that:

a. government intervention is necessary to deal with externalities. b. there are no costs associated with pollution abatement. c. when there are external costs, the industry supply curve reflects the true social costs. d. when externalities are present, an economy can reach an efficient solution as long as transactions costs are not too high.

What is the key assumption about a consumer's marginal rate of substitution?

What will be an ideal response?