Fact 1: Dilation occurs when cell walls relax.

Fact 2: An aneurysm is a dilation of a blood vessel.

Given the two facts above, answer the question "What relaxes, thereby causing aneurysms?" with the following options: 
- layers of fat
 - Veins and arteries.
 - sweating
 - matter vibrating
 - blood vessels
 - the skin
 - Chemical energy
 - sensory neurons


 

     blood vessels    