For each of the following studies, identify the source(s) of sampling bias and describe (i) how it might affect the study conclusions and (ii) how you might alter the sampling method to avoid the bias.
(a) Eight hundred volunteers were recruited from nightclubs to enroll in an experiment to evaluate a new treatment for social anxiety.
(b) In a water pollution study, water specimens were collected from a stream on 15 rainy days.
(c) To study the size (radius) distribution of scrub oaks (shrubby oak trees), 20 oak trees were selected by using random latitude/longitude coordinates. If the random coordinate fell within the canopy of a tree, the tree was selected; if not, another random location was generated.