What security risk does a covert channel create?

A.
A process can signal information to another process.

B.
It bypasses the reference monitor functions.

C.
A user can send data to another user.

D.
Data can be disclosed by inference.

Explanation:
The risk is not that a process can signal another process. The risk is that the signaling bypasses the
reference monitor functions (ie the communication is not screened by the security kernel that
implements the reference monitor).