The Shelf Life of a Battery Produced by One Major

Multiple Choice

The shelf life of a battery produced by one major company is known to be Normally distributed, with a mean life of 3.5 years and a standard deviation of 0.75 years. If a sample of size 20 is repeatedly chosen at random, what is the standard deviation _x of sample means?

A) 3.5 years
B) 0.75 years
C) 0.17 years
D) 0.04 years