You are presented with the quiz "What feature of many cacti help them survive in dry environments?" 

But you don't know the answer, so you turn to your teacher to ask for hints. He says 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". 

So, what's the best answer to the question?


 

   Expandable stems        