Which of the Following Is an Example of a Matched

Which of the following is an example of a matched pairs design?

A) Migraine-suffering patients receive aspirin one week and a new painkiller combination another week. The treatment order is randomized.
B) One set of teenagers get a topical fluoride application, while their twins don't. Which twin gets the fluoride treatment is randomly assigned for each twin pair.
C) In a study of diet and genetics, researchers assign randomly two rat pups from the same litter to be fed either a high-fat diet or a high-fiber diet. In all, the study used a dozen rat litters.
D) All of these answers are correct.