Mrs. Boyd surveyed every 4th teacher coming into the building about the average amount of time a middle school student spends on social media. The survey described did not produce valid conclusions due to bias. Which of the following sampling methods should Mrs. Boyd have used for it to be unbiased a Survey the student with the highest GPA in each grade level.
b Survey Ms. Lawson's fourth period math class.
c Survey every 4th student as they enter school.
d Send an email to parents with the survey attached.