"Stratified Sampling: Suppose that the student population below is divided into four strata (base on some criterion). The table lists the students' names and the strata to which they are assigned. A sample of 70 students is needed.
a) How many students are in each stratum?
b) How many students from each stratum should be selected for a sample?
c) Using stratified sampling method, select an actual sample of 70 students."