A polling agency is employed to investigate the voting intention of residents in a particular electorate. From the data collected, they want to predict the election result for that electorate in the next election. Explain why each of the following situations may produce a biased sample:
a. a A random selection of people in the local large shopping complex is surveyed between 1 pm and 3 pm on a weekday.
b. b The members of the local golf club are surveyed.
c. c A random sample of people at the local train station between 7 am and 9 am are surveyed.
d. d A door to door visit is undertaken, surveying every voter in a particular street.