What is one of the main differences between the phosphorus and sulfur cycles? a) plants absorb phosphorus mainly from the air and sulfur mainly from the soil. b) animals get phosphorus by eating plants and sulfur from the air they breathe. c) human activities contribute only to the amount of sulfur in the atmosphere but not to that of phosphorus. d) the atmosphere has no significant role in the phosphorus cycle, but is an essential part of the sulfur cycle.

The right answer is D.

The phosphorus cycle is unique among the major biogeochemical cycles because it does not have a gaseous component; it hardly affects the atmosphere.

The sulfur cycle is the biogeochemical cycle of the different forms of sulfur. Sulfur is an essential element of life that, like carbon, phosphorus, oxygen, nitrogen or even water, has its own life cycle. Indeed, sulfur is present everywhere on Earth, atmosphere, oceans, continents, but also in all living beings in the form of organic molecules: sulfur-containing amino acids constituting proteins (methionine and cysteine).

the answer is true when observing or determining population size, a direct observation is use, in which each organism is counted in a population or in an area. one benefit of this measure is that you get the exact population count but it will take a long time to  determine  the  size  of a  population.