Algebra 2

in a sample of 200 households, the mean number of hours spent on social networking sites during the month of january was 55 hours. in a much larger study, the standard deviation was determined to be 7 hours. assume the population standard deviation is the same. what is the 99% confidence interval for the mean hours devoted to social networking in january?

a) the 99% confidence interval ranges from 53.73 to 56.27 hours.
b) the 99% confidence interval ranges from 7 to 55 hours.
c) the 99% confidence interval ranges from 54.51 to 55.49 hours.
d) the 99% confidence interval ranges from 55 to 60 hours.

can i get an explanation?
all i know is that this question has something to do with the margin of error.
the margin of error has two different formulas
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