Are chemical signals that, instead of being secreted into the bloodstream, are introduced by animals into their habitats. they can be used to attract mates and mark territories.

explanation:

b,bacteria convert nitrates

into nitrogen gas.

d,plants and animals take in

and use nitrogen compounds.

e,bacteria break down waste    

products and return nitrogen  

to the soil.

a,bacteria convert atmospheric

nitrogen to ammonia.

c,bacteria convert ammonia

to nitrites and nitrates.

i think it is dna, i'm not totally sure but i think it might be.