问题如下：
When the maturity of a plain coupon bond increases, its duration increases
选项：
A.Indefinitely and regularly
B.In a way dependent on the bond being priced above or below par
C.Indefinitely and progressively
D.Up to a certain level
解释：
ANSWER: B
For bonds with coupons but higher YTM, the price of such bonds will be more discounted. Its duration should be to rise first and then fall again.
For bonds with coupons but higher YTM——这里错了吧
应该是对于 lower YTM吧， 对于高YTM的bonds，溢价发行，不会出现 上升再下降的情况，只有折价发行才会如此 。
