Using step-deviation method, calculate the mean marks of the following distribution.

[4] Class interval | Frequency
50-55 | 5
55-60 | 20
60-65 | 10
65-70 | 10
70-75 | 9
75-80 | 6
80-85 | 12
85-90 | 8

A) 71.25
B) 72.5
C) 73.75
D) 75