Sample A
The rate of change for a horizontal line would be zero because the y-values are constant, while the x-values change and zero divided by any non-zero number equals zero.

Sample B
I drew a horizonal line and picked two points (0, 5) and (4, 5) to find the change in independent and dependent variables.  5-5 divided by 4-0 is zero divided by four which is still zero.  The rate of change of the line I drew is zero, so I think the rate of change of a horizontal line will be zero, but it might be different with a different line.

Sample C
I think a horizonal line goes from side to side and not up and down, if so this would mean that the dependent variable was not changing , so I think the rate of change will be zero.

Teacher Comments
This exit slip was used as a critical thinking exercise after an initial lesson on finding rates of change from graphs.  No horizonal or vertical lines had been used as examples or in practice exercises in class.  Most students were able to extend and generalize the concept to include graphs of horizonal lines.