How can we clear data in combobx2 on selecting next item in

I used two combo boxes cmbhospitals and cmbusers in cmbusers when i click hospital A in 1st combbox it displays users of that hospital but again when i click on hospital B in cmbusers(Combobox2) it was displaying both users og hospital A and B I used if which clears the text but not the previous hosptal data please suggest.

 

I used below three to clear but getting the same old values when selecting.

 

//cmbhusers.SelectedItem=null;

cmbhusers.Items.Clear();

cmbhusers.ResetText();

cmbhusers.SelectedIndex=-1;