Why are two separate bins provided, for collecting garbage, in some cities?

• A. It helps the city to be categorized in a better city.
• B. These are provided to collect separately the materials those which do not rot into garbage heaps and the wastes that rot completely when buried in soil.              
• C. These bins are in different colours and attracts us to put the garbage in these dustbins.
• D. None of the above is correct.