单项选择题

Passage Three
Mobile phone recycling is turning into big business for some kids in India. They were once street wanders, but now they are businessmen. The kids have been digging old mobile phones out of the rubbish and selling them to recycling centres. They now live in a small house, are able to go to school and they have food. These kids are marvelous and they are probably the future business leaders of their town. The mobile phone recycling industry is growing rapidly around the world as newer models come out and more people want to dispose of their old phones. Every day there seems to be another story about the damage we have done to the environment. Mobile phones contain thing like lead, mercury（水银）and other harmful material s that can kill us if it reaches our water supply and is not treated properly. Mobile phone recycling keeps these phones out of wasteyards and water supplies. Recycling centers are appearing all overt the world to tear things down, separate out the useable materials and then burn the rest of the material for energy. This is a complete purposeful use for the abandoned items, and the same holds true for mobile phone recycling. The precious metals inside these phones are reused so that moiré does not have to be mined for manufacturing purposes. This saves time, money, resources and gets rid of the wastes all at the same time.

The mobile phone recycling industry grows rapidly mainly because （）.