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. Given these facts, what feature of many cacti help them survive in dry environments among the following options:
- exoskeleton
 - Spines for hunting
 - Expandable stems
 - an external framework
 - cactuses
 - Cooling pipes
 - genetic diversity
 - Loud organ pipes


 

   Expandable stems      