4. At the very beginning of the study, researchers interviewed university graduates. How long did it take for them to be part of the study once again? a) A decade-long. b) A period of 8.5 years. c) It is not mentioned.

c) It is not mentioned.<br /><br />The question does not provide any specific information about the duration it took for the university graduates to be part of the study again. Therefore, the correct answer is "It is not mentioned."