Burt, a Mentally Impaired Person, Is Asked by Carl to Sign

Burt, a mentally impaired person, is asked by Carl to sign a piece of pa?per that Carl says is an autograph book. In fact, the document is a note. If later sued by an HDC, Burt's best defense would be

A) duress.
B) mistake.
C) fraud in the inducement.
D) fraud in the execution.