State the null hypothesis Ho and the alternative hypothesis Ha that would be used for a hypothesis test related to each of the following statements:
a. The mean age of the students enrolled in evening classes at a certain college is greater than 26 years.
b. The mean weight of packages shipped on Air Express during the past month was less than 36.7 l b.
c. The mean life of fluorescent light bulbs is at least 1600 hours.
d. The mean strength of welds by a new process is different from 570 lb per unit area, the mean strength of welds by the old process.