How can I determin the chemical herbicide content in these fish?

Question

A random sample of 20 fish were taken from the day's catch of fish in Toxic Bay and the mean level, m, of a chemical herbicide in their blood was found to be 20 parts per million with a standard deviation of 1.2 parts per million. Find the 99.5% confidence interval of the true mean.



a. 19.6 < m < 20.4

b. 19.5 < m < 20.5

c. 19.4 < m < 20.6

d. 19.3 < m < 20.7

e. 19.2 < m < 20.8

f. 19.1 < m < 20.9

g. none of these

Answer 1

I'd say g. Since we have the sd of the mean as 1.2, even 2 sd, which defines the central 95% of the true mean would be from 18.8 to 21.2