Tom looks out his window. Which observation includes a biotic factor of the ecosystem he is in. A. It is raining.
B. There are worms on his sidewalk.
C. The water in his pond is higher than usual.
D. The clouds are darker than they had been a little earlier.

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.

organ system-an organ system is a group of organs that work together to perform one or more functions. each does a particular job in the body, and is made up of certain tissues.