Which is TRUE regarding the employment at-will doctrine in the United States?

a. It can result in a legal but immoral discharge of an employee.
b. It cannot be changed by a contract between the employer and the employee.
c. It is operable in the U.S. and can result in a discharge without notice or cause or good cause but the employer based on federal law has to pay severance to the discharged employee based on the number of years he or she has worked for the firm.
d. It supersedes and prevails over Title VII of the Civil Rights Act.