The last one has never been used in practice yet ; the second one is used under strict conditions ; and the first one is used in every comer of the world . we should use the purchase method in all combinations 重新開始法在全球還沒得到實際應用,權益集合法在少數(shù)國家(如美國)得到了有限的應用,購買法是應用最廣的一種方法。