A design engineer wants to construct a sample mean chart for controlling the service life of one of the types of light bulbs 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 bulbs, with these results: Sample 1 2 3 495 525 470 SERVICE LIFE (hours) 500 505 500 515 505 515 480 460 470 If he uses upper and lower control limits of 520 and 480 hours, what is his risk (alpha) of concluding service life is out of control when it is actually under control (Type I error)? Select one: a. 0.0026 b. 0.3174 c. 0.6826 d. 0.9544