Which of the following are true about various security models? a. The Graham-Denning model operates on a set of subjects, a set of objects, a set of rights and an access control matrix b. Bell- LaPadula is a security model that allows for data of different levels of sensitivity c. Bell-LaPadula is a formalisation of a security policy d. The Graham-Denning model does not allow rights to be transferred from one subject to another.