If Researchers Using Matched Control Subjects Find That Abused Children

If researchers using matched control subjects find that abused children are sadder than nonabused children, those researchers know that:

A) both groups of children showed equal levels of sadness before the study.
B) the nonabused group differed from the abused group in many important ways.
C) there were more girls than boys in the abused group because girls are more likely to be sad.
D) abuse is probably what is causing the difference in sadness between these groups.