while looking at a graph, you notice a period of time where the line is perfectly horizontal what is most likely taking place during this time period

The correct answer would be - the y-axis variable is constant.

Explanation:

In a normal line graph, there are generally two variables on two different axis one is at the x-axis and the other is on the y-axis that represents the correlation between these two variables. If there is a change in the variable present on the x-axis the change on the variable on the y-axis establishes the relationship among these variables.

If there is a period on the graph where the line is completely horizontal it means the variable present on Y-axis has no change during this period or is constant. Suppose a graph is plotted between speed or velocity on the Y-axis. A perfectly horizontal line on this graph means that speed is not changing over time and remain at te same speed.

links? do you have a link? i don't see anything?

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