Piston Rings for an Automotive Engine Are Produced by a Forging

Piston rings for an automotive engine are produced by a forging process.We wish to monitor the inside diameter of the rings manufactured by this process using an and an s control chart.Samples of size 4 are to be taken at regular intervals,and the sample means and standard deviations are computed and plotted on the charts in time order.The target values for the inside diameter are a mean of $\mu$ = 75 millimeters and a standard deviation of $\sigma$ = 0.02 millimeters.The control chart constants are (partially) reproduced below. What is the center line for the
Chart?

A) 0.03 millimeters
B) 74.97 millimeters
C) 75 millimeters
D) 75.03 millimeters