Some life forms move so slowly that their movement cannot be detected by normal observation. The best way to determine any movement is by placing a mark at some point, and observing any change in position from it. Why is this true?

Movement means a change in position relative to a reference point. By marking a location and checking whether the life form’s position changes relative to it, you can detect slow movement that is hard to see directly. Therefore, “the mark is a reference point” is the correct choice.

AI-written and checked against the verified answer.