Six samples of five observations each have been taken for a process. The sample means and ranges computed (in minutes) are, (mean, range): (3.6, 0.66), (3.51, 0.47), (4.07, 0.58), (4.12, 0.59), (3.64, 0.67), and (4.11, 0.46). Compute the upper control limit

a.

2.8

b.

4.116

c.

4.173

d.

2.416