Which of the following statements are true of ozone?

a. It is necessary for the manufacturing of nitrogen fertilizers and aerosol sprays.
b. It is necessary for the upper atmosphere to absorb the deadly ultraviolet radiation of the sun.
c. It is a radioactive gas that produces ultraviolet radiation and is used for the treatment of cancer.
d. It is a rare form of oxygen that is poisonous to human beings at ground level.