数据验证使用列表填充清空的单元格

I have a sheet where user can only enter data based on data validation drop down list, ( the list contain "Yes", "NO" "N/A")is there any way through VBA or Data validation, fill the empty cell with the last item in the list ("N/A")when the workbook opens and keep the Data Validation format and delete the ("N/A") when the workbook close.I have a sheet where user can only enter data b