Answer:
B. "The algae decrease the amount of oxygen in the water, resulting in a 'dead zone' that is inhospitable to fish and other organisms."
Step-by-step explanation:
The author of this passage explains how farming has had a negative effect on the river. He tells us that farming has altered the way algae grows. As algae becomes more pervasive, it decreases the amount of oxygen in the water. Algae grows quickly, and becomes a plague. As the oxygen decreases, the environment becomes inhospitable to fish and other organisms.