Explain why the following will not lead to random samples from the desired populations.

(a) To determine what the average person spends on a vacation, a market researcher interviews passengers on a luxury cruise.

(b) To determine the average income of its graduates 10 years after graduation, the alumni office of a university sent questionnaires in 2016 to all the members of the class of 2006 and based its estimate on the questionnaires returned.

(c) To determine public sentiment about certain import restrictions, an interviewer asks voters: "Do you feel that this unfair practice should be stopped?"