You are presented with the question "what is a cactus stem used for storing?" and the following answer choices: 
- H20
 - carbon monoxide
 - CO2
 - vinegar
 - rain
 - O2
 - sunlight
 - h2o

Now knowing that acactus stem is used for storing water and water is made up of H2O molecules, choose the best answer.


 

        h2o 