A is true. It is affected by changes in the structure of the economy such as changes in the rate of population growth, the rate of technological change. B is false. It is not a constant and tends to vary as the economy is subject to shocks. C is true. They may have different views of the growth rate, expected inflation etc. That's why it is not directly observable. The rate has to be estimated usually with a lot of uncertainty has to be estimated, usually with a lot of uncertainty.

The price of all units of output sold must decrease.

This source of risk is unique to the firm.

For a 90% confidence interval we find z(0.05), the cut-off for the top 5% of the normal distribution. Looking up 0.95 in the middle of the table the reading to the row/column values, we get 1.645.
Working with the formula for E (see top right) we get E = 0.1. So, the 90% confidence interval is 1.3 - 0.1 m 1.3 + 0.1 or 1.2 m 1.4.