What conclusion do Desjardins and Duska reach about drug testing in the workplace?

What conclusion do Desjardins and Duska reach about drug testing in the
  workplace?

a. Employees are not forced to work for corporations that require drug testing
  as a precondition of employment. Since drug testing is fully voluntary, it is
  morally permissible.
  b. Drug testing is permissible because drug users decrease productivity and
  pose an increased risk for workplace accidents.
  c. Drug testing is a violation of privacy and is therefore categorically wrong.
  d. Privacy is an employee right, and drug testing is compatible with this right
  only if the information it seeks is relevant to the employment contract.