Please help me with a detailed explanation and a distinguisher attack on this scheme! Thank you, I will rate!

Suppose Enc-then-MAC +AD is instantiated with CBC mode and any secure MAC, as described in the text. The scheme is secure for fixed-length associated data. Show that if variable-length associated data is allowed, then the scheme does not provide AEAD security. Note: you are not attacking the MAC! Take advantage of the fact that dc is ambiguous when the length of d is not fixed and publicly known. For reference: this question is refering to the AEAD security (definition 15.2 in the book) of this scheme: The set of associated data signifiers is D={0,1}