You are presented with the question "What feature of many cacti help them survive in dry environments?" and the following answer choices: 
- exoskeleton
 - Spines for hunting
 - Expandable stems
 - an external framework
 - cactuses
 - Cooling pipes
 - genetic diversity
 - Loud organ pipes

Now knowing that conserving water can be used for survival in a dry environment and many cacti have corrugated stems which can expand quickly if water becomes available, choose the best answer.


 

   Expandable stems      