George killed his two children because he believed he heard voices telling him that if he did not kill them, his neighbor would. His defense attorney argued that due to George's insanity at the time of the crime, George should not be held responsible for the crime and should thus be acquitted of the crime. Which of the following best describes the plea that George's defense attorney made?
a. Not Guilty but Mentally Ill
b. The Durham test
c. Not Guilty by Reason of Insanity
d. Guilty But Mentally Insane