C#DataGridView中的 DataGridViewComboBoxColumn 如何在加載窗體時設置默認顯示值SelectedValue，或時，在新增加一行時，顯示默認的值？謝謝。

            DataGridViewComboBoxColumn cd = new DataGridViewComboBoxColumn();        
            cd.HeaderText = "基税定額";
            cd.Name = "PersonalTaxThreshold";           
            cd.DataSource = personalTaxThresholdListBLL.SelectPersonalTaxThresholdListIsDataTable(personalTaxThresholdIs);
            cd.DisplayMember = "PersonalTaxThresholdAmount";
            cd.DataPropertyName = "PersonalTaxThreshold";
            cd.ValueMember = "PersonalTaxThresholdUid";
            cd.DisplayIndex = 1;            
            cd.MaxDropDownItems = 10;
            dataGridView1.Columns.Add(cd);

如何在dataGridView1 中DataGridViewComboBoxColumn顯示默認值？還不沒有選擇ComboBox時，顯示默認值。