Question

fill in the blank question
you can tell that the boat is in motion using the

as a reference point because the distance between the two objects changes.
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Explanation:

To determine if an object (the boat) is in motion, a reference point is used. A reference point is a stationary object (like a tree, shore, or a fixed object in the environment) against which the position of the boat is compared. If the distance between the boat and this reference point changes, the boat is in motion. Common reference points for a boat could be the shore, a dock, a tree on land, or a stationary buoy. For example, if we use the shore as a reference point, as the boat moves, the distance between the boat and the shore changes, indicating motion.

Answer:

shore (or other stationary objects like dock, tree, buoy etc. - a typical answer here is a stationary object relative to the boat's movement, with "shore" being a common and simple example)