The number of phone calls arriving at a switchboard each hour has been recorded, and the following frequency distribution has been developed.

Class Interval              Frequency

20-under 40                   30

40-under 60                   45

60-under 80                   80

80-under 100                 45

What is the relative frequency of the second class?

A. 0.45

B. 0.90

C. 0.225

D. 0.75