C. Why do people use Earth's motions to
determine units of time?

The people use Earth's motion to determine units of time because of its uniform motion which is consistent.

Explanation:

In earlier days, people used to observe the motion of Earth to measure time. This is because, the Earth is moving at constant speed and at constant acceleration. So the change in position of the sun or stars can be easily correlated to the time.

Due to the consistency in the Earth's motion and the accurate repetition of the time period along with the seasons, the Earth's motion can be considered as a responsible source to determine the units of time.

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