Clearing child Dropdownlist items by unselecting Parentdropdown

Hi Friends,
Below is the code for, when you select a checkbostlist(A)  item say X: ,you get all items of X in another checkboxlist (B):
Similarly when you select a checkbostlist(A)  item say Y: ,you get all items of Y in another checkboxlist (C):
 
Protected void cblFacilities_SelectedIndexChanged( ...