An experimenter conducts an experiment on the effects of a drug to control hallucinations. he declares the results to be "statistically significant" which usually means that

the differences between experimental and control groups occur only by chance.

Explanation:

In an experiment, the result is said to be statistically significant or to have statistical significance, if it is likely not caused by chance for a given statistical significance level. So, if an experimenter conducts an experiment on the effects of a drug to control hallucinations. he declares the results to be "statistically significant" which usually means that the differences between experimental and control groups occur only by chance

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.

the answer is c on e2020, bud.