The numbers of hours worked (per week) by 400 statistics students are shown below.

Number of Hours		Frequency
0 x < 10			20
10 x < 20			80
20 x < 30			200
30 x < 40			100

The class width for this distribution _____.

a. is 10

b. is 9

c. varies from class to class

d. is 40, which is the largest value minus the smallest value or 40 0 = 40