问题如下：
Which type of sovereign bond has the lowest interest rate risk for an investor?
选项：
A. Floaters
B. Coupon bonds
C. Discount bonds
解释：
A is correct.
Floaters are bonds with a floating rate of interest that resets periodically based on changes in the level of a reference rate, such as Libor. Because changes in the reference rate reflect changes in market interest rates, price changes of floaters are far less pronounced than those of fixed-rate bonds, such as coupon bonds and discount bonds. Thus, investors holding floaters are less exposed to interest rate risk than investors holding fixed-rate discount or coupon bonds.
discount bound是什么？讲义里有讲过么？