The lifespan of brand A batteries is normally distributed with a mean of 108 hours and a standard deviation of 12.5 hours. The lifespan of brand B batteries is normally distributed with a mean of 92 hours and a standard deviation of 15.1 hours. A quality-control specialist selects a random sample of 25 of each type of battery and calculates the sample mean lifespan for each brand. What is the mean of the sampling distribution of the difference in sample means? Do not round your answer.
a) 0
b) 15
c) 16
d) 20