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The correct answer is B.

OBJ-2.4: NIST’s SP 800-63-3 recommends that SMS messages be deprecated as a means of delivering a second factor for multifactor authentication because they may be accessible to attackers. SMS is unable to be encrypted (at least without adding additional applications to phones). A third factor is typically not a user-friendly recommendation and would be better handled by replacing SMS with the proposed third factor. SMS is not a costly method since it can be deployed for less than $20/month at scale.

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