Fact 1: Light enters the eye through an opening called the pupil.

Fact 2: Light enters the eye through the pupil, and the pupil is made larger or smaller by the iris.

Given the two facts above, answer the question "What makes the pupil change sizes?" with the following options: 
- electrically
 - sun's heat
 - sweating
 - It expands
 - The iris
 - nerves
 - environment
 - heat


 

     The iris    