为什么第三个是错的？第五个是对的啊？

问题如下：

Which of the following statements are TRUE?

I.The convexity of a 10-year zero coupon bond is higher than the convexity of a 10-year, 6% bond.

II.The convexity of a 10-year zero coupon bond is higher than the convexity of a 6% bond with a duration of 10 years.

III.Convexity grows proportionately with the maturity of the bond.

IV.Convexity is always positive for all types of bonds.

V.Convexity is always positive for “straight” bonds.

选项：

A.I only B.I and II only C.I and V only D.II, III, and V only

解释：

All else equal, convexity increase for longer maturities, lower coupons, and lower yields.

Bonds with embedded options (e.g., callable bonds) exhibit negative convexity over certain ranges of yields while straight bonds with no embedded options exhibit positive convexity over the entire range of yields.