Asubstance that enters earth's cycles and does harm to living things is
a. oxygen.
b. freshwater.
c. a pollutant.
d. hydrogen.

The best answer is c - pollutant

 A substance that enters the earth's cycles and does harm to living things is called a pollutant. It is a substance  that renders air, water and land unsafe and unsuitable for use.

Some pollutants can decay (biodegradable) and therefore will not stay in the environment for long. Others are not biodegradable, especially some man made materials.

The factors that determine how severe a pollutant is are mainly the chemical nature of the pollutant, its concentration and how long it can persist  in the environment without decaying.

are there any answer choices

answers are:

lysogenic     viral dna merges with host cell dna

lysogenic     no symptoms

lytic               viral dna takes over

lysogenic     virus does not destroy host cell

lytic               virus destroys host cell

lytic               induces symptoms of viral infection

lysogenic       cell reproduces and spreads provirus