Please check the only INCORRECT statement of Malkiel's duration rules

	a.	The duration of a zero coupon bond is higher than the duration of a coupon bond of the same yield and time to maturity
	b.	The duration of a perpetual bond trading at a yield of 5% is 21 years
	c.	Holding the coupon rate constant, the duration of a coupon bond is lower when the bond's yield to maturity is lower
	d.	Holding the maturity constant, a bond's duration is higher when the coupon rate is lower