Dr. Lawrence is the director of Counseling Services at her university. She is planning to conduct a survey of 100 students at the university to see how aware they are of the counseling services that are offered at the university. She wants the proportion of first-time students and returning students in her sample to reflect the proportion in the university as a whole (15% first-time and 85% returning). Dr. Lawrence plans to stand in the Student Union and ask people to participate until she has given the survey to 15 first-time and 85 returning students. Which type of sampling method is Dr. Lawrence going to use