you wonder if tv ads are more effective when they are longer or repeated more often or both. so you design an experiment. you prepare 30-second and 60-second ads for a camera. your subjects all watch the same tv program, but you assign them at random to four groups. one group sees the 30-second ad once during the program; another sees it three times; the third group sees the 60-second ad once; and the last group sees the 60-second ad three times. you ask all subjects how likely they are to buy the camera. which of the following best describes the design of this experiment?