Answer: The reasonable units that could measure how fast is Horace running, are mi/h and km/h.
Explanation:
That is because the velocity is defined as the ratio between the change in the length in an interval of time. We can write this algebraically as:
Velocity (or the same "how fast is Horace running") = ΔL/Δt
So, in that way, Horace could measure his velocity just with a units tan can stablish a big difference of length on the time. For example, it would be easy for him to measure his velocity since the time that he expends while he runs one kilometer. Otherwise, it would be very difficult for him to measure his velocity if he just runs one-millimeter o one-inch, because it will be so difficult too, to measure the time that he expends running that distance