dropdwon list - user must select an item other than -SELECT ONE-, before submitting the page - use of validator control

In my asp.net 2.0 page, I placed a dropdwon list
the list is filled from the datatabase in formload() event, and I added one more entry -SELECT ONE-
This select one will be displayed when the page loads. Ok fine.
I want the user to select an item other than -SELECT ONE-, before submitting the page,
ex: list shows country names, user should select a country name.
Which validator control is to be used for this, and how can i code this.
any idea

 
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