Which of the following biometric characteristics cannot be used to uniquely authenticate an individual's identity?

The following are typical biometric characteristics that are used to uniquely authenticate an individual's identity:

Fingerprints

Retina scans

Iris scans

Facial scans

Palm scans

Hand geometry

Voice

Handwritten signature dynamics

Source: KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten Domains of Computer Security, 2001, John Wiley & Sons, Page 39.

And: HARRIS, Shon, All-In-One CISSP Certification Exam Guide, McGraw-Hill/Osborne, 2002, chapter 4: Access Control (pages 127-131).