Models need to be changed when a. they are based on experimental data b. scientific understanding changes c. they represent a phenomenon d. they are designed to answer a question select the best answer from the choices provided a b c d

the answer is a because if the model is based on experimental data it wont be good unless it has been tested.

B

Explanation:

scientific understanding changes

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.