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:

A reference point is needed to detect motion. The mark helps Julie see if the snail changes position relative to it. It's not for determining speed initially, nor is it a standard unit of measure, and displacement requires more than just a starting - point check for motion detection.

Answer:

acting as a reference point for detecting motion