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.