When homeowners plant trees, the fallen leaves can create a __________ externality, imposing cleanup costs on neighbors that are not reflected in the market price of the trees.

When homeowners plant trees, the leaves that fall can create a negative externality. This means that the benefits of having trees, like shade and beauty, come with costs that affect others, such as neighbors who have to clean up the fallen leaves. These cleanup costs are not included in the price of the trees, so the homeowners do not pay for the trouble their trees cause to their neighbors. For example, if a homeowner has a large oak tree that drops a lot of leaves, their neighbors might spend extra time and money raking and disposing of these leaves. This situation shows how one person's choices can unintentionally impact others in the community.