Question 638353
A)

Correct


B)

This should be closer to -0.782460796.


Note: sqrt(30) is NOT 900


C)

The difference between the two is that the standard deviations aren't the same. In the first, the standard deviation is simply sigma. In the second, the standard deviation is sigma/sqrt(n) -- this is known as the standard error of the mean. This affects the shape/spread of the distribution, which in turn affects how far the data value is from the mean. This then changes/affects the z-score.