Just need help in understanding why this answer is C.

Thanks in advance.

A consumer organization wants to estimate the actual tread wear index of a brand name of tires that claims"graded 150" on the sidewall of the tire. A random sample of n=15 indicates a sample mean tread wear index of 136.7 and a sample standard deviation of 29.5. Complete parts(a) through(c).

Explain why an observed tread wear index of 153 for a particular tire is notunusual, even though it is outside the confidence interval developed in(a).

A. It is not unusual because it is just outside the confidence interval.

B. It is not unusual because it is actually in the confidence interval.

C. It is not unusual because it is only 0.55 standard deviations above the sample mean.

Your answer is correct.

D. It is not unusual because it is only 0.10 standard deviations above the confidence interval.