How to bind or populate data into a Dropdown List based on first or another Dropdown List in Asp.net C#

In many forums i found this problem that's why i decide to write a post on populating a Dropdown List based on another Dropdown List. Here i will try to show how we can bind sql server data with a Dropdown List as well as after selection populate other Dropdown List. To describe this situation here i will use a very common scenario like country & its regions. Lets we have a country table of ID,