The main disadvantage of Access Control Matrix (ACM) is that:
a. ACM, with a large number of users and objects, can become really huge, and most users of the system may not have a need to access most objects in the system.
b. ACM cannot be used for authorization
c. ACM, with a large number of users and objects, can become really small, making it very hard to maintain its integrity.
d. ACM is rather a password hashing method; not really used for authorization.