Final answer:
Counting the number of oxygen bubbles can give you the amount of O2 because plants release oxygen gas as a byproduct during photosynthesis.
Step-by-step explanation:
The number of oxygen bubbles can be used to determine the amount of O2 because during photosynthesis, plants release oxygen gas as a byproduct. The release of oxygen gas leads to the formation of bubbles. Therefore, counting the number of oxygen bubbles gives an indication of the amount of oxygen produced.