A less-than cumulative frequency table for marks obtained by 30 students is given below:
Less than 10: 6
Less than 20: 15
Less than 30: 20
Less than 40: 28
Less than 50: 30
Which of the following is the median class?
(a) 20–30
(b) 40–50
(c) 10–20
(d) 30–40