A guard is commonly used with a classified system. What is the main purpose of implementing and using a guard?

A.
To ensure that less trusted systems only receive acknowledgments and not messages

B.
To ensure proper information flow

C.
To ensure that less trusted and more trusted systems have open architectures and interoperability

D.
To allow multilevel and dedicated mode systems to communicate

Explanation:
B: The guard accepts requests from the less trusted entity, reviews the request to make sure it is allowed, and then submits the request on behalf of the less trusted system. The goal is to ensure that information does not flow from a high security level to a low security level in an unauthorized manner.