Yes, I thought so too, but in the EOC, there is no mention of any options. And the exhibit provide Eff.Dur and Mod.Dur values and I’m still a bit confused why the answer used Eff.Dur in the calculation.
I think Effective Duration measure is also more precise and, if you’re able so, rather use ED instead MD. MD is based on Macaulay and is old-fashioned measure, imo.