4. A chemical or physical agent in the environment that causes a mutation is called a
A. leucism.
B. point mutation.
C. mutagen
D. polyploidy

Mutagen

Explanation:

In genetics, a mutagen is a physical or chemical agent that changes the genetic material, usually DNA, of an organism and thus increases the frequency of mutations above the natural background level. 

significant amounts of nitrogen oxides, carbon monoxide, and other pollution

explanation:

passenger vehicles are a major pollution contributor, producing significant amounts of nitrogen oxides, carbon monoxide, and other pollution. in 2013, transportation contributed more than half of the carbon monoxide and nitrogen oxides, and almost a quarter of the hydrocarbons emitted into our air.

each of the 46 original chromosomes splits into two daughter chromosomes, so there are two sets of 46 daughter chromosomes that end up in each cell. human cells have 46 chromosomes. each chromosome consists of a pair of identical chromatids attached together by a structure called a centromere.