The number of phone calls arriving at a switchboard each hour has been recorded and the following frequency distribution has been developed.
What is the cumulative frequency of the third class？（）

A.a graph of a frequency distribution
B.a running total of the frequencies in the classes
C.the proportion of the total frequencies which fall into each class
D.a very cloudy picture of the frequencies

A.histogram
B.ogive
C.stem and leaf chart
D.pie chart

A.the number of classes must be less than 10
B.each data point can only fall into one class
C.the width of each class is equal to the range
D.the number of intervals must be an odd number