Which of the following yields a stratified sample?

a.All students in a class are divided into groups of 15. One student is randomly chosen from the first group, the remaining observations are every 15th student thereafter.

b.The best 15 students, according to the opinion of the instructor, in a class are selected.

c.All students in a class are grouped according to their gender. A random sample of 8 is selected from the males and a separate random sample of 7 is drawn from the females.

d,\. The first 15 students in a class are selected without replacement.

Which of the following yields a simple random sample?

a.All students in a class are divided into groups of 15. One student is randomly chosen from the first group, the remaining observations are every 15th student thereafter.

b.The best 15 students, according to the opinion of the instructor, in a class are selected.

c.The names of 50 students in a class are written on 50 different pieces of paper and put in a hat. The first 15 pieces of paper are selected blindly one at a time without replacing them back in the hat after shuffling the papers thoroughly.

d. All students in a class are divided into groups according to the rows in which they are seated. One of the groups is randomly selected