Classify each sample as random, systematic,stratified, or cluster.
a. In a large school district, all teachers from two buildings are interviewed to determine whether they believe the students have less homework to do now than in previous years.
b. Every seventh customer entering a shopping mall is asked to select her or his favorite store.
c. Nursing supervisors are selected using random numbers to determine annual salaries.
d. Every 100th hamburger manufactured is checked to determine its fat content.
e. Mail carriers of a large city are divided into four groups according to gender (male or female) and according to whether they walk or ride on their routes. Then 10 are selected from each group and interviewed to determine whether they have been bitten by a dog in the last year.