Deanna has been hired to visit the local shopping mall to conduct a survey about the upcoming political election. She needs to select respondents at the mall and ask them questions about their voting tendencies. Suppose that Deanna decides to randomly select four stores in the mall and then approach each shopper in these stores and ask them to participate in the survey. Which of the following sampling techniques best describes Deanna's method?
a. systematic
b. simple random
c. probability
d. convenience