Recycling Resources (资源再生) Increased population increases the need for natural resources and also increases waste products．Making a new useful product out of a waste product decreases our need for natural resources． Recycling means using something over again after it has been used once. Have you ever seen a piece of recycled paper? About one third of all the paper products made each year are made from waste paper．For example, recycled paper is usually used to make paper sacks． 'Recycling paper, such as newspapers, reduces the number of trees that must be cut each year to make new paper. Each person produces a lot of solid waste．In the United States, the average is eight kilograms of waste per person each day．Most of the waste is garbage and rubbish．Finding places to dispose of solid waste is a major problem. In many cities solid.wastes are collected and the rubbish is burned in incinerator plants (焚化厂)．Air pollution control devices are used to prevent pollution Heat produced in the burning may be used to generate electricity．Waste that is not burned is taken to a dump． One kind of dump for the disposal of solid wastes is a sanitary(卫生的)landfill（垃圾填埋场）． This method involves burying the waste and bulldozing (用推土机推) soil over them． Parks, baseball fields, airports, and other developments may be buiIt on completely filled landfills, Recycling solid wastes decreases our need for methods of disposal．The metal in broken cars can be recycled and made into new steel．Worn out rubber tires can be turned into new ones and used again．Plastics and other rubber products can be recycled．Metal cans and glass bottles can also be recycled．Increased recycling can help meet the increased demand for natural resources. By recycling resources, people imitate nature．Elements important to life are naturally recycled．Nothing lasts forever．Wooddecays, iron rusts (生锈), and concrete is worn away by wind and rain．All natural resources used by people eventually go back into the air, water, and earth. 第 41 题 What is the main cause of the increased waste, according to the passage?