Renu is planning a study on recycling attitudes. She obtains a list of all neighborhoods in her city. She decides to select three neighborhoods to survey. She chooses her own neighborhood and two nearby neighborhoods because it will be convenient for her to distribute surveys in these areas. Next, in each neighborhood, Renu picks out the first two streets from a list arranged in order of the cost of houses on each street (that is, the street with the most expensive houses is listed first, then the one with the second highest priced houses, and the least affluent street is listed last). She decides to send a questionnaire by mail to each house on each selected street. Her budget only allows her to send out one questionnaire per house. Here are some of Joann's questions: a. Briefly describe one possible threat to external validity in Renu's design. b. Is Renu's sample a probability (i.e., random) sample? Please answer "yes" or "no" and then explain in a sentence or two why or why not. c. In a few sentences at most, please give Renu one suggestion of how she can improve her study design.