A National Manufacturer of Ball Bearings Is Experimenting with Two $\pm$

Multiple Choice

A national manufacturer of ball bearings is experimenting with two different processes for producing precision ball bearings. It is important that the diameters be as close as possible to an industry standard. The output from each process is sampled and the average error from the industry standard is calculated. The results are presented below.
The researcher is interested in determining whether there is evidence that the two processes yield different average errors. Assume that the population standard deviations are equal. What is the critical t value at the 1% level of significance?

A) +2.779
B) -2.492
C) $\pm$ 1.711
D) $\pm$ 2.797