Fact 1: Caffeine is a central nervous system stimulant.

Fact 2: Caffeine is a stimulant, and a stimulant constricts blood vessels.

Given the two facts above, answer the question "What does caffeine do to blood vessels?" with the following options: 
- It expands
 - looseness
 - hydrate their cells
 - Constrict them
 - sweating
 - Dehydration
 - They may not survive.
 - Damages them


 

    Constrict them     