Phytoremediation is the use of plant growth to purify pollutants from soil, water, or air. suppose that a crop of brake ferns can remove 15 milligrams(mg) per square meter of a particular pollutant from the soil in 20 weeks. after 20 weeks, the ferns are harvested and a new crop is planted. if c represents the number of crops of brake ferns needed to phytoremediate soil contaminated with170 mg per square meter of the pollutant down to healthy levels of 5 mg per square meter, which equation best models the situation? a) 170 - 20c = 5 b) 170 + 20c = 5 c) 170 - 15c = 5 d) 170 + 15c = 5