Provide brief explanations or comments on the following statements (a) It is impossible to offer proper access control

[answerhappygod]

Provide brief explanations or comments on the following
statements
(a) It is impossible to offer proper access control within
applications that allow the use of pointer (e.g., C applications)
because pointers can be made to point to any location in memory
thus bypassing access control.
(b) Since key length is critical for the security of block
ciphers we can therefore conclude that long keys guarantee the
security of encrypted messages.
(c) In a MLS security scheme a user cleared for
(TOPSECRET, {B, C}), wants to access a document repository
classified (TOPSECRET, {A, B, C, E, D}). What sort of access is
allowed?
(d) If certificate authorities cannot be trusted, then
public key protocols (such as TLS) cannot be reliably used.