Writing the alternative hypothesis as the population mean being less than 16.43 makes the test:

1. The form of the alternative sets which tail of the distribution matters. 2. A statement that the mean is less than 16.43 is the alternative hypothesis in one worked case. 3. It points in a single direction, so only the lower tail counts as evidence against the null. 4. A two-sided alternative would use a not-equal-to statement instead. _Source: OpenStax Introductory Statistics 2e (CC BY 4.0), Ch 9 'Hypothesis Testing with One Sample', sections 9.1-9.6_