When u. s. farmers in the southwest irrigate their land, salt in the ground soil leaks into the colorado river. the colorado river has become so salty that mexican farmers further down the river cannot irrigate their own land and mexican crops have been devastated. this situation constitutes a negative externality because:
a. the social cost of irrigation exceeds the social benefits.
b. u. s. farmers consider only their external costs in their decision making.
c. mexican farmers consider only their private costs in their decision making.
d. u. s. farmers consider only their private costs in their decision making.