What Is "Elephant Snot

What is "elephant snot?"

A) Mucus produced by people who have had significant exposure to atmospheric pollution.
B) The slimy covering that blankets trees and other plants in areas that are exposed to high levels of sulfur dioxide.
C) Filamentous green algae that grows on the bottom of lakes that have experienced acidification.
D) The build-up of decayed fish and other aquatic animals that die and drop to the bottom of lakes, streams, or rivers after the pH has dropped due to the effects of acid rain.