It is known that an average of 12 patients arrive at a health clinic. every hour. To verify the accuracy of this information, 22 hours are monitored on di erent days. For each hour, the number of arriving patients is determined. These 22 measurements are used to produce a 90% confidence interval for the mean number arrivals in one hour assuming the population is normally distributed. Below is the output of R which gives us this interval:

What is the standard deviation of this sample.

> t.test(x,conf.level = 0.90)$conf.int

[1] 12.14 15.28

A) 4.28 B) 5.28 C) 2.68 D)3.54 E)12.14