Economic activities, though grouped into three different categories, are highly interdependent. Raw materials are produced in Primary sector and processed into finished items in Secondary sector. Assistance is provided by Tertiary sector to these two activities. Lets take an example of iron-ore. It is extracted in mining which is a Primary activity, then it is transported to industries for making pig-iron and steel in iron and steel plant, which is a Secondary activity. Transportation and finance services are provided to these two sectors by Tertiary sector.