listbox conection

Question

Hi to all, i have started only recently to use the c# language and i have encountered a problem which i hope you might be able to help me solve.i have made a form which connects to a data base with 3 tables,one containing the world's continents,one filled in with soe countries randomly selected and one with cities.When i run the class the contents of the 1st table gets loaded in a list box.I need after i select 1 continent from the list box one more listbox to be displayed containing the countries of that continent.I am stuck..i know it has sth to do with the primary keys of the DB and the list box attributes but i can't think of sth so i would really appreciate any help given, thank you in advance alexander