A histogram with equal class width 5 has been drawn for the weights of 40 students. The measured heights of the rectangles (in cm) are:
Class 40–44 : 4 cm
Class 45–49 : 6 cm
Class 50–54 : ? cm
Class 55–59 : 2 cm
The vertical scale used is 1 cm = 2 students. Find the frequency of the class 50–54.
(a) 12
(b) 16
(c) 8
(d) 18