A student recently wrote us: "This question is extremely unclear. It is very difficult to discern that the black line is the only one that functions in units of MPa. On your feedback graph, it is clear that most students made the mistake of of selecting C due to only considering the gray line."
Line graphs with double y axes are definitely tricky to interpret. Whenever a graph like this is presented, it is important to carefully read the figure legend in order to determine what each line on the graph represents. Reading the legend, we see that it clearly states the black line represents compressive strength (MPa), which is what the question is asking for. On the other hand, the grey line represents the mean CECT attenuation.
Thanks for your question. I hope this helps!
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