A Salon and Spa Chain Periodically Analyzes Its Service Times

A salon and spa chain periodically analyzes its service times to check for variation in service processes. To do this, it uses R-charts. Seven samples, each containing observed service time with three different customers, were collected. The average mean and the average range of the service times are found to be 9.924 and 0.957, respectively. If D 4 for the R-chart is 1.924, the upper control limit ( ) for the R-chart will be:

A) less than or equal to 1.0.
B) greater than 1.0 but less than or equal to 1.5.
C) greater than 1.5 but less than or equal to 2.0.
D) greater than 2.0 but less than or equal to 2.5.