The Numbers of Hours Worked (Per Week) by 400 Statistics $$\begin{array} { l l } \text { Number of hours } & \text { Frequency } \\0 - 9 & 20 \\10 - 19 & 80 \\20 - 29 & 200 \\30 - 39 & 100\end{array}$$

Multiple Choice

The numbers of hours worked (per week) by 400 statistics students are shown below. $$\begin{array} { l l } \text { Number of hours } & \text { Frequency } \\0 - 9 & 20 \\10 - 19 & 80 \\20 - 29 & 200 \\30 - 39 & 100\end{array}$$ The class width used in this frequency distribution is

A) 4.5.
B) 9.
C) 10.
D) 39.