What does the simple security (ss) property mean in the Bell-LaPadula model?

What does the simple security (ss) property mean in the Bell-LaPadula model?
A .  No read up
B .  No write down
C .  No read down
D .  No write up

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Answer: A

Explanation: The ss (simple security) property of the Bell-LaPadula access control model states that reading of information by a subject at a lower sensitivity level from an object at a higher sensitivity level is not permitted (no read up). Source: KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten Domains of Computer Security, John Wiley & Sons, 2001, Chapter 5: Security Architectures and Models (page 202).