Asp.net and Java Script quesstion

I have a web form with a gridview control on it. The GridView has a column of check boxes. Before the web form closes I need to check if more than
one check box was checked. If more than one check box was checked, then I need to display a popup message box informing the user that
only one check box should be checked?
I need to check to see if more than one check box was checked, when a "Submit" button on the form is pressed.
How can this be done using Asp.net/C# and Java Script?
 
I know how to iterate through rows of a Grid control so I can do that in the C# code. But how to keep the web form open before I iterate through the rows, I don't know?
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