There is no diffrence in the nitrogen content of inspired and expired air. Suggest a reason for this.

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

sheep is a herbivore animal whereas grasshopper belongs to the class chordates but both of them feed on greens.  

if grasshopper and sheep feeding on a pasture land, i do not think both of them will have any impact on each other. however, the number of the species can make difference. since sheep feed on greens, there is a possibility there left no pasture land for the grasshoppers or they may get extinct.