红框里是怎么取值算的啊，为啥要减去？还有d(0.5) d(1) d(1.5)的含义分别是什么？

问题如下：

The table below gives coupon rates and mid-market price for three U.S. Treasury bonds for settlement on (as of) May 31, 2013

Which of the following is nearest to the implied discount function (set of discount factors) assuming semi-annual compounding?

选项：

A.

d(0.5) = 0.9370, d(1.0) = 0.8667, d(1.5) = 0.9210

B.

d(0.5) = 0.9920, d(1.0) = 0.9700, d(1.5) = 0.9350

C.

d(0.5) = 0.9999, d(1.0) = 0.7455, d(1.5) = 0.8018

D.

d(0.5) = 1.0350, d(1.0) = 1.1175, d(1.5) = 0.6487

解释：

The future value of $1 invested for time t is 1/d(t).