Choose the correct statement, according to the passage above:

   Life on earth has always been exposed to a certain amount of radiation from natural sources, which comes in part from cosmic rays. Also present in our environment is radiation from the radioactive elements that occur naturally such as radium, thorium, and uranium.

   One of the most serious problems that can result from radiation given off by nuclear reactions is damage to the genes, the cells in living organisms that control hereditary factors. Exposure to radiation can cause a mutation, a change in some inherited characteristics of the organisms. Only a very small proportion of mutations are beneficial to the organism.

   The nuclear reactors that have been developed for experimental, peaceful and energy production purposes are surrounded by a heavy protective shield of concrete, or sometimes of lead, to protect the people who work with them.

  A further problem involves the disposal of radioactive wastes. As the number of nuclear reactors in existence has increased, so has the amount of dangerous waste products. They cannot simply be released into the environment because they could create a potential hazard for all living things in the world. Experimentation is trying constantly to find safe methods of getting rid of the harmful by-products of nuclear reactors. (p. 75)

Adaptado de LIBERATO, Wilson Antônio. De olho no vestibular (Inglês -Textos). São Paulo: FTD, 1996. 144 p.