A design engineer wants to construct a sample mean chart for controlling the service life of a halogen headlamp his company produces. On three recent production batches, he tested service te on random samples of four headlamps, with these results: Service Life (hours) 1 2 3 4 495 500 505 500 Sample 1 Sample 2 Sample 3 525 515 505 515 470 480 460 470 What are the x-bar chart three-sigma upper and lower control limits? Select one: O a. 520 and 480 b. 507.2 and 4828 O c. 499.2 and 478.2 Od. Incomplete information to calculate the limits O e. 503.7 and 478.3