Dropdow control

In my web page i have three dropdown control.

1 for Country, 2nd for State and 3rd for City.

now i want like when i select a country say india then dropdown-state must show states of india only.

and i select USA then states of USA should be inside state-dropdownlist.

 and after that city-dropdown should show the city of selected state.

so please tell me how to do this.
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