What is the iris? the protective layer over the eye
the nerve at the back of the eye
a muscle that reacts to light by contracting
it controls color blindness

the colored part of the eye which helps regulate the amount of light entering the eye

Explanation:

the Iris uses muscles to change the size of the pupil.

exposure to hazardous carcinogens and toxic substances can cause breathing problems, such as copd and various types of cancer. it also increases the risk of heart attacks and strokes and has serious (but no less unattractive) side effects, such as aging, yellow teeth, a terrible smell and damage to the vocal cords.