In Which I Pose a Fairly Easy Question from Alcuin, the Monk

Alcuin was head teacher in the palace school of Charlemagne at Aachen. Here is one of the questions in his Propositiones ad acuendos iuvenes:

An ox plows all day long. How many tracks does he leave in the last furrow?

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Answer: None. The plow removes them all.

(HT: Percival Blakney Academy)