Which of the following statements about eutrophication is true?
a.

eutrophication occurs when excess nutrients are present in aquatic ecosystems, resulting in the increased production of plant life and the subsequent increase in the oxygen levels of the water.
b.

eutrophication occurs when excess nutrients are present in aquatic ecosystems, resulting in the increased production of plant life and the subsequent decrease in the oxygen levels of the water.
c.

eutrophication involves the overpopulation of aquatic ecosystems with plant and animal life.
d.

eutrophication is rarely caused by human activity.