The Number of Phone Calls Arriving at a Switchboard Each

The number of phone calls arriving at a switchboard each hour has been recorded, and the following frequency distribution has been developed.
$$\begin{array} { l c } \text { Class Interval } & \text { Frequency } \\20 \text {-under } 40 & 30 \\40 \text {-under } 60 & 45 \\60 \text {-under } 80 & 80 \\80 \text {-under } 100 & 45\end{array}$$
What is the relative frequency of the second class?

A) 0.455
B) 0.900
C) 0.225
D) 0.750
E) 0.725