Monovalent and bivalent OPVs respectively contain one or two types of attenuated poliovirus, reducing the number of strains that could possibly mutate.

和二口服脊髓灰质炎疫苗分别含有种或两种毒脊髓灰质炎病毒，少了可能发生突变的毒株数量。

The law of one price means that in the absence of transportation costs and official trade barriers, it is agreed that the price of a commodity in different regions should be the same for the same currency.

定律意味着，在没有运输成本和官方垒的情况下，人们致认为，同种货币在不同地区的商品格应该相同。