Shelley Is a Researcher Who Studies Disabilities Resulting from Head

Shelley is a researcher who studies disabilities resulting from head injuries.She has chosen to use non-experimental methods because of some of the limitations of experiments.Which limit would most likely cause problems for Shelley's research

A)  It is not ethical to conduct experiments with people with disabilities. 
B)  People cannot be randomly assigned to a group that experiences a head injury. 
C)  Disability cannot be operationally defined. 
D)  Experiments cannot be used to study interaction effects.