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

1 495 500 505 500

2 525 515 505 515

3 470 480 460 470

What is the mean of the sampling distribution of sample means for whenever service life is in control?

A) 250 Hours

B) 470 Hours

C) 495 Hours

D) 500 Hours

E) 515 Hours