Ulcers are sores on the lining of the digestive tract, caused by the bacteria Helicobacter pylori. You want to test the effectiveness of two new treatments for stomach ulcers. Out of 750 people with stomach ulcers, who are willing to serve as subjects, the researchers record various response variables and a number of demographic variables. For example, it was found that 100 subjects are under 18 years old, 250 subjects are 18-35 years old, 250 subjects are 35-55 years old, and 150 subjects are older than 55 years. Researchers speculated that people of different age groups would not react differently to the treatments. a. completely randomized design. b. randomized block design. c. matched pairs design.