A design engineer wants to construct a sample mean chart for controlling the service life of a halogen headlamp his company produces. He knows from numerous previous samples that this service life is normally distributed with a mean of 500 hours and a standard deviation of 20 hours. On three recent production batches, he tested service life on random samples of four headlamps, with the following results: What is the standard deviation of the sampling distribution of sample means for whenever service life is in control? Sample SERVICE LIFE (hours. 1 495 500 505 500 2 525 515 505 515 3 470 480 460 470 Select one: A. 10 hours B. 6.67 hours C. 11.55 hours D. 5 hours E. 20 hours