Certain species of what are now recognized as indicators of environmental pollution

The correct answer is lichens.

Explanation:

Lichens are a symbiotic association between the fungi and algae. The lichens are bioindicators, that is, they are very subtle to atmospheric pollution like nitrogen as they obtain all their water and nutrients from dry and wet atmospheric deposition. The deposition of nitrogen can enhance the nutrients load.  

Some of the species of lichens are more forbearing to nitrogen in comparison to others. The communities of lichens are monitored by the scientists, that is, if an enhancement in nitrogen tolerant species in association with a reduction in nitrogen sensitive species takes place, it suggests that there is an elevation in atmospheric nitrogen deposition.  

cuz they cant breath

explanation:

stress fractures occur largely in the feet and legs and are common in sports that require repetitive impact, primarily running/jumping sports such as gymnastics or track and field.