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. (a) This is a randomized block design, but not a matched pairs design. (b) This is a matched pairs design. (c) This is a completely randomized design with one explanatory variable (factor). (d) This is a completely randomized design with two explanatory variables (factors). (e) This is a completely randomized design with four explanatory variables (factors).