Six samples of five observations each have been taken for a process. The sample means and ranges computed (in minutes) are, (mean, range): (3.57, 0.51), (3.75, 0.46), (3.74, 0.72), (3.86, 0.78), (4, 0.43), and (3.93, 0.41). Compute the upper control limit

a.

4.128

b.

4.073

c.

2.38

d.

2.76