Critical case sampling refers to research that selects those cases that can have a decisive impact on things, with the goal of logically inferring the results obtained in these cases to other cases. which of the following are key cases? ['E-mail survey of all customers who purchase a product to understand where the product can be improved.', 'Interviews with several 6-year-olds to eliminate unpopular candy flavors developed specifically for 6-year-olds.', 'Develop a strict telephone sampling plan, requiring investigators to call customers to collect feedback on product packaging.', 'Randomly find objects in the residential area that meet the product fragrance survey, and invite the test to complete a questionnaire.']