The U.S. generates about four billion tons of solid waste a year. More than 90 percent is

composed of agricultural and mining wastes. About 3 percent is from industrial wastes. 250 million tons of garbage and refuse come out from homes, schools, office buildings, stores and hospitals.

This mass of metals, paper, food, plastic, rubber and glass which is all mixed together seems of no earthly use to anyone.

Or is it？ Scattered efforts across the country indicate that new technology, properly applied, might someday turn these millions of tons of refuse into an excellent source of raw materials for new uses.

Several new projects are carried out to study the possibility of treating wastes through pyrolysis (高温分解)--a system of reducing refuse to basic chemicals, liquids and gases which have commercial value. In Florida, a special tube system provides automatic transport of refuse to a central collection building.

In short, many new ideas are being tested, and many will be needed, since no one method is likely to prove suitable everywhere.

However, many difficult economic and social questions must be answered before real progress can be made. Can recovered materials compete with new materials？ What will be needed as encouragement to make resource recovery work？ Will citizens pay the costs of changing our waste system？

The article suggests that garbage and refuse from homes and public places ______.

A．make up the largest percentage of solid wastes

B．make up a small percentage of solid wastes

C．are not really considered in the solid waste problem

D．are considered of no use to the earth