Edward has just finished marking the group assignment today. The distribution of the marks looks like a normal distribution. Suppose that the summary statistics show that the median mark is 15, which is also equal to the mean and the mode. Which of the following is correct? Select one: O a. none of the listed answers is correct. b. the interquartile range is 0 O c. the interquartile range equals the arithmetic mean O d. the range and interquartile range are equal