Examine each of the following situations and state whether you would expect to find a high correlation between the variables. Give reasons why an observed correlation cannot be interpreted as a direct relationship between the variables and indicate at least one possible lurking variable.
(a) The correlation between the number of Internet users and truck sales in cities of varying sizes.
(b) The correlation between yearly sales of cell phones and number of new credit cards issued over the past 10 years.
(c) Correlation between the concentration x of air pollutants and the number of riders y on public transportation facilities when the data are collected from several cities that vary greatly in size.