The Number of Phone Calls Arriving at a Switchboard Each

Multiple Choice

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 midpoint of the last class?

A) 80
B) 100
C) 95
D) 90
E) 85