为什么求D1不用foreign- domestic

问题如下：

The current exchange rate for a currency is 1.2000 and the volatility of the exchange rate is 10%. Calculate the value of a call option to buy 100 units of the currency in two years at an exchange rate of 1.2500. The domestic and foreign risk-free interest rates are 3% and 5%, respectively.

解释：

In this case, S0 = 1.2000, K = 1.2500, r = 0.03,

rf = 0.05, u = 0.1, and T = 2, and Equation (15.10)

gives

d,=$\frac{\ln(1.200/1.2500)\;+\;(0.03\;-\;0.05\;+\;0.10^2/2)\;X\;2}{0.1\sqrt2}$= -0.5008

d2 =$\frac{\ln(1.200/1.2500)\;+\;(0.03\;-\;0.05\;-\;0.10^2/2)\;X\;2}{0.1\sqrt2}$= -0.6422

C = 1.2OOO$e^{-0.05\ast2}$N(-O.5OO8) - 1.25OO$e^{-0.03\ast2}$N(-O.6422)= 0.028

This is the value of an option to buy one unit of the currency. The value of an option to buy 100 units is 2.8.