What are the main environmental issues associated with the phosphorus cycle

answer:

we remove phosphates on certain locations, to apply it on farmland as fertilizers. on these locations the phosphate supply is moving, causing phosphate levels of surrounding land-soils and groundwater to become too high.   on farmland where phosphates are applied as fertilizers, plants do not absorb all phosphate. the phosphates end up in water and stream towards lakes and reservoirs, where they cause a phenomenon called eutrophication.   as a result the oxygen supply in the water depletes, causing fish and other oxygen-dependent organisms to die and bacteria that are not oxygen dependent (anaerobic) to take over.   when people practise extensive wood chopping in forest areas, such as the tropical rain forest, phosphate supplies deplete, because most of the phosphate can be found in the ground under the trees in such areas. when the trees are removed the rain washes phosphates away, causing the ground to become unproductive.   phosphate supplies are restored very slowly, because most phosphate in soils and water comes from weathering rocks and oceanic sediments.

i think a) betsy can donate only blood to mark is correct because, mark and betsy have the same blood type. whereas nobody else has b+ blood so betsy can't donate to them.

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."