单项选择题

Land pollution involves many kinds of wastes. For many years, all wastes were dumped 61 sanitary (卫生的) landfills. Sanitary landfills are large pits where garbage is buried under layers of dirt. When hazardous wastes are put in these landfills, toxic (有毒的) substances can seep into the groundwater and enter the food chain. Hazardous wastes are those kinds of wastes that are 62 or dangerous to the environment. They can be poisonous, corrosive (腐蚀性的), flammable, explosive, or radioactive (放射性的). They can pollute the air or 63 fires or explosions. These wastes can also cause 64 problems for humans and animals. For these reasons, it is important to dispose 65 hazardous wastes in secured landfills where they cannot leak. A secured landfill is located on clay ground, and the pits are lined with plastic and nylon sheets to 66 the hazardous wastes in the pit. One kind of hazardous wastes, radioactive waste, involves a special disposal problem. Radioactive waste is created by industries and nuclear power plants 67 use radioactive materials. Radioactive materials 68 off energy as their atoms change. This energy is invisible, but very powerful. It can harm 69 tissues in plants and animals. Radioactive waste can remain hazardous for over 100 years. They must be stored in containers that can hold them without leakage for at least 70 amount of time.