Fact 1: Conserving water can be used for survival in a dry environment.

Fact 2: Many cacti have corrugated stems which can expand quickly if water becomes available.

Given the two facts above, answer the question "What feature of many cacti help them survive in dry environments?" with the following options: 
- exoskeleton
 - Spines for hunting
 - Expandable stems
 - an external framework
 - cactuses
 - Cooling pipes
 - genetic diversity
 - Loud organ pipes


 

   Expandable stems      