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, we use a reference point (a stationary object, like the shore, a tree, or a dock). When 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 buoy, or a stationary object on land/water. A typical answer here is "shore" (or other stationary objects like a tree, dock, etc.).

Answer:

shore (or other stationary objects like tree, dock, buoy; the key is a stationary object whose distance to the boat changes)