单项选择题 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 approximate range of the number of phone calls arriving each hour？（）

单项选择题 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？（）

单项选择题 A cumulative frequency distribution would provide（）.