Which of the Following Studies Used a Single-Factor, Matched Groups

Which of the following studies used a single-factor, matched groups design?

A) Kroeger et al.'s study, which evaluated the effects of a social skills program for autism
B) the study by Mueller and Oppenheimer, which compared groups taking two types of class notes
C) Boothby et al.'s study comparing the effects of the shared experiences of eating chocolate or viewing art
D) the study by McDonald and Flanagan, which examined the effects of traumatic brain injury on understanding social exchanges