A.
it is enough to know only one exchange rate to determine the third (只知道一个汇率就可以确定第三个汇率)
B.
we can estimate two exchange rates based on one exchange rate only (我们只能根据一种汇率估算两种汇率)
C.
it is necessary to know all three rates (有必要了解全部三个比率)
D.
it is enough to know the exchange rates between any two in order to determine the third (知道任意两个之间的汇率就足以确定第三个)