Many naturally occurring environmental exposures increase risk for cancer. residents in many parts of north america are exposed to low levels of the radioactive gas in their homes, particularly in homes with basements.

RADON GAS

Explanation:

Radon is a naturally occurring gas, which is produced due to the radioactive decay of the Radium element. It is one of the dense gas that is mostly concentrated in the lower portion of the houses, and are trapped in the cracks and fractures of the walls of houses and buildings.

Radon gas is an invisible, colorless and odorless gas, which is a very harmful gas, and exposure to Radon gas can cause an individual to suffer from cancer.

In recent studies, it has been found that in many parts of North America, a large number of residents are exposed to Radon gas, which led to an increased risk of cancer.

complete the square on the right side of the equation;

-5(x + 0)^2 + 15

answer: d. y = -x^2 + 15