It Is Suggested by the Authors That Reference Checks to Previous

It is suggested by the authors that reference checks to previous employers by phone

A) Are useless because previous employers should not make negative comments about previous employees because of the "liberty clause".
B) Are good if you ask questions that a previous employer can safely answer such as "Would you hire this person again"?
C) Are generally not helpful because you don't have any information in a written form..
D) Are beneficial because you can learn a great deal from a previous employer verbally that often they would not be willing to put in a written form.