Delete rows from Google Sheets based on multi column criterea

How would I be able to achieve this ??
I want to delete all rows having the SAME value in Column D, where Value in Column B = Value in Column C

function onFormSubmit() {
ss = SpreadsheetApp.getActive();
sh = ss.getSheetByName('Form responses 1');
rg = sh.getDataRange().getValues();
if ( ??????????????????) {
sh.deleteRows(????????????); //deletes all rows having same Coln D, where Coln B = Coln C
}
}