Fact 1: Constriction occurs when the muscular walls of blood vessels contract.

Fact 2: Dehydration causes vessels to constrict and blood to thicken.

Given the two facts above, answer the question "What causes blood to thicken?" with the following options: 
- Constriction
 - evaporation
 - heat or cold
 - thirst
 - permineralization
 - sweating
 - flow of electrons
 - matter vibrating


 

 Constriction        