Explain whether each of the following is a positive or negative externality.
Smoking on campus exposes students to secondhand smoke.
• College graduates add to the nation's supply of skilled workers and add to productivity.
• Biotech firms invent new medicines.
• Using energy-saving light bulbs reduces the pollution from electricity-generating plants.
• Crowded freeways with heavy truck usage expose the surrounding communities to increased health risks.
• Restoring the Amazon forest can reduce global levels of pollution.