A statistics student who is curious about the relationship between the amount of time students spend on social networking sites and their performance at school decides to conduct a survey. Various research strategies for collecting data are described below. In each, name the sampling method proposed and any bias you might expect.

(a) He randomly samples 40 students from the study's population, gives them the survey, asks them to ll it out and bring it back the next day.

(b) He gives out the survey only to his friends, making sure each one of them lls out the survey.

(c) He posts a link to an online survey on Facebook and asks his friends to ll out the survey.

(d) He randomly samples 5 classes and asks a random sample of students from those classes to ll out the survey.