Jack, Jill and Bob each own a piece of land next to each other. Each of their property is located next to a swamp, which emits a foul odor. Therefore, Jack proposes that they all pay to remove the swamp. However, only Jack and Jill end up paying for the removal of the swamp, while Bob enjoys his newly swampless property. The problem identified above is known as: An externality problem A contracting problem A subsidy problem A free-rider problem A framing problem None of the above