Which of the following statements about a class interval is correct?
(a) If two class intervals have equal class size, their class marks must be equal.
(b) The class mark of an interval is always exactly half of its class size.
(c) The difference between the class mark and the lower class limit equals half of the class size.
(d) The class size of an interval is obtained by adding 1 to the difference between its upper and lower class limits.