Without seeing the actual double bar graph you are referring to, I can't provide a definitive answer. However, you can determine what the vertical axis represents by looking for labels or information on the graph itself. Generally, if the vertical axis is measuring how much has been read (in terms of either pages or books), the options would likely focus on one of those measurements.
If the graph measures reading volume in terms of pages, options A or B would be plausible; if it measures in terms of books, option C would be correct. The increment (like 5 or 10) can also typically be identified from markings on the vertical axis.
Please examine the graph closely to find the corresponding labels, and that should help you conclude the correct answer.