The type of reproductive barrier that occurs when two species mate but produce sterile hybrids is referred to as

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.

there is no

explanation:

if your own plato the answer is b. the fern transitioned from sporophyte generation to gametophyte generation.

the answer to your question is tooth decay.