Question

julie finds a snail on the sidewalk and wants to know whether or not the snail moves throughout the day. she places a single mark on the sidewalk next to the snail.
what will julie use the mark for initially?
determining how fast the snail moves
acting as a standard unit of measure
determining the displacement of the snail
acting as a reference point for detecting motion

Explanation:

The goal is to check if the snail moves. A fixed mark next to the snail serves as a fixed point to compare the snail's position over time. The other options require additional measurements (speed needs time/distance, unit of measure is a standard scale, displacement needs final position relative to start, but the initial use is just to detect if position changes vs the mark).

Answer:

acting as a reference point for detecting motion