问题如下：
Suppose investors have interest rate expectations as illustrated in the decision tree below where the 1-year rate is expected to be 8%, 6%, or 4% in the second year and either 7% or 5% in the first year for a zero-coupon bond.
If investors are risk-neutral, what is the price of a $1 face value 2-year zero-coupon bond today?
选项：
A.$0.88113.
B.$0.88634.
C.$0.89007.
D.$0.89032.
解释：
Assuming investors are risk-neutral, the following decision tree illustrates the calculation of the price of a 2-year zero-coupon bond using the expected rates given. The expected price in one year for the upper node is $0.93458, calculated as $1 / 1.07. The expected price in one year for the lower node is $0.95238, calculated as $1 / 1.05. Thus, the current price is $0.89007, calculated as:
[0.5 x ($0.93458 / 1.06)] + [0.5 x ($0.95238 / 1.06)] = $0.89007
请问这个题为什么用不到second year的8%呢？为什么直接从first year 开始算？
