error in processing Drop Down List

Following error comes when I processes a drop down list.
[HttpException (0x80004005): Cannot have multiple items selected in a DropDownList.]

My code is folloeing

foreach (ListItem list in Period_of_course.Items)
                {
                    if(list.Text == myReader["Period_of_educational_course"].ToString().Trim())
                    {
                   list.Selected=true;
                       break;
        &nbs ...