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The compressive strength of concrete is normally distributed with mean = 1200 psi and standard deviation-30 A random sample of n-9 specimens will have a sample mean diameter that falls in the interval from 1011 psi to 1230 psi. What is the standard error of the sample mean?
A. 400/3
B. None of the others
C. 40
D. 30/9
Ε. 10
