All other things equal (YTM = 10%), which of the following has the longest duration?

All other things equal (YTM = 10%), which of the following has the longest duration?
A. a 30-year bond with a 10% coupon
B. a 20-year bond with a 9% coupon
C. a 20-year bond with a 7% coupon
D. a 10-year zero-coupon bond