Astatistics teacher wants to know how her students feel about an introductory statistics course. she decides to administer a survey to a random sample of students taking the course. she has several sampling plans to choose from. which plan is simple random sampling?

a) there are four grade levels of students taking the class: freshmen, sophomores, juniors, and seniors. randomly select 15 students from each grade level
b) randomly select a grade level (freshmen, sophomores, juniors, and seniors) and survey every student in that grade level
c) each student has a nine-digit studentnumber. randomly choose 60 numbers.
d) using the class roster, select every fifth student from the list.