State the null and alternative hypotheses that would be used to test the following claims:
a. The difference between the means of the two populations is more than 20 lb.
b. The mean of population A is less than 50 more than the mean of population B.
c. The difference between the two populations is at least $500.
d. The average size yard for neighborhood A is no more than 30 square yards greater than the average size yard in neighborhood B.