A sociologist studying the difference in ages between husbands and wives obtained a random sample of 55 married couples. The mean of the husbands’ ages was 38.5 years with standard deviation 12.6 years. The mean of the wives’ ages was 36.9 years with standard deviation 12.4 years. The sociologist calculated the difference between the ages for each couple. The mean difference was 1.6 years with standard deviation 2.1 years. A matched-pairs hypothesis test will be performed to investigate whether the difference is significant. Which of the following is the standard error for the test statistic for the hypothesis test?