The latest project is to take a city of around half a million inhabitants and discover exactly what raw materials go into it and what go out. The aim is to find out how much of these raw materials could be provided if a plant for recycling waste were built just outside the city. This plant would recycle not only metal such as steel, lead and copper, but also paper and rubber as well.
Another new project is being set up to discover the best ways of sorting and separating the rubbish. When this project is complete, the rubbish will be processed like this: first, it will pass through sharp metal bars which will tear open the plastic bags in which rubbish is usually packed; then it will pass through a powerful fan to separate the lightest elements from the heavy solids; after that grounders and rollers break up everything that can be broken. Finally the rubbish will pass under magnets, which will remove the bits of iron and steel; the rubber and plastic will then be sorted out in the final stage.
The first full scale giant recycling plants are, perhaps, fifteen years away. Indeed, with the growing cost of transporting rubbish to more distant dumps, some big cities will-be forced to build their own recycling plants before long.
The main purpose of the passage is ______.

A. to show us a future way of recycling wastes
B. to tell the importance of recycling wastes
C. to warn people the danger of some wastes
D. to introduce a new recycling plant