experiment yeast fermentation ，how do the results show that fermentation has taken place in boiling tubeA？​

The bubble formation is stopped.

Explanation:

The results show that fermentation has taken place in boiling tube A when the bubble formation is stopped which is the indication that the fermentation process is completed. If the bubbles are continue to produce from the solution this means that the fermentation process is taking place and not yet completed. These bubbles are formed due to production of carbondioxide in the reaction which provides information about fermentation process.

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.