Random Samples of 4 Items Are Selected from a Manufacturing $\Sigma$

Multiple Choice

Random samples of 4 items are selected from a manufacturing process at regular intervals and a certain quality characteristic is measured. After 25 samples, $\Sigma$ X = 15,350 and $\Sigma$ R = 411.4
To construct a mean chart, what is the value for R bar used to compute the limits?

A) 411.4
B) 614
C) 16.46
D) 102.88