Which of the Following Is the Most Usable Definition of Law

Which of the following is the most usable definition of law?

A) It is most useful to us to define law in terms of what we think people ought to do.
B) We must define law in terms of what people used to do.
C) We must define law in terms of what the courts or other agents of government will enforce.
D) We must define law in terms of what society needs people to do.
E) It is most useful to us to define law in terms of what people do.