Fact 1: Dilation occurs when cell walls relax.

Fact 2: When oxygen level decreases, dilatation of the blood vessels occurs.

Given the two facts above, answer the question "What causes cell walls to relax?" with the following options: 
- exercise
 - sweating
 - adding heat
 - Energy
 - dilation
 - heat or cold
 - heat
 - Dehydration


 

     dilation    