Suppose that on a hypothesis test for a single population mean, $H_a : \mu \lt 10$. Assume that Ha is true. For a fixed sample size and significance level $\alpha$, the power of the test will be greatest if the actual mean is which of the following?

The test has greater power when the probability that the alternative hypothesis is true is greatest. If the true mean is 8, then the probability that the mean is less than 10 is quite large.