All of the following statements are correct except ______.
选项：

A:trading on a documentary bill basis offers more protection than trading on the basis of clean bills of exchange
B:to trade on a documentary bill basis, the documents are handed over to the importer only against acceptance or payment of the bill
C:to trade on a clean bill basis, the documents are handed over to the importer only against acceptance or payment of the bill
D:to trade on a documentary credit basis, the drawer is assured of acceptance and payment by the issuing bank