In this case you need find column which having unique values or multiple columns which together behave unique. for example you can combine a column and b column then it will give a unique value which help you to identify rows uniquely.
When binding you can set both display and value members. As value member you can bind b column value.
then you can change the select statement as below
"select * from test where a =@ddlText and b =@ddlVal order by cylinder_no asc"
To avoid sql injection attacks, you better use parameterized sql statements
set @ddlText parameter value as DropDownList2.Text and @ddlVal parameter value as DropDownList2.SelectedItem.Value