Footer Template filed's visible is set to false

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C#

C#4.0

In my Gridview Footer i have DropDown list and TextBox and LinkButton . In my dropDown I have some fields

in that one of the field is "-N/A-" value of that field is "7".

Now my doubt is when i select the DropDown Value " -N/A-" then the remaining fields like TextBox and LinkButton visiblity to set as false .

protected void GV_RowCommand(object sender, GridViewCommandEventArgs e) 
    { 
       if (e.CommandName == "Add") 
        { 
            GridView grid = (GridView)sender; 
            DataGridItem item = grid.NamingContainer as DataGridItem; 
             
            int Doc_No = Convert.ToInt32(gdDrawing.DataKeys[item.ItemIndex]); 
            ViewState["Doc_Id"] = Doc_No; 
            string Material = (grid.FooterRow.FindControl("ddlMaterial_Type") as DropDownList).SelectedItem.Text; 
            int Material_Id = Convert.ToInt32((grid.FooterRow.FindControl("ddlMaterial_Type") as DropDownList).SelectedValue); 
            string Quantity = (grid.FooterRow.FindControl("txtMaterialQty") as TextBox).Text; 
 
 
           //material_id = 7 then textbox and linkbutton of footer to set visible as false 
} 
}

can any one help me to solve this issue