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.