Fact 1: Acactus stem is used for storing water.

Fact 2: Water is made up of H2O molecules.

Given the two facts above, answer the question "what is a cactus stem used for storing?" with the following options: 
- H20
 - carbon monoxide
 - CO2
 - vinegar
 - rain
 - O2
 - sunlight
 - h2o


 

        h2o 