Which of the Following Assumptions Are Not Made by Classification

Which of the following assumptions are not made by classification systems for psychological disorders, such as DSM-5 and ICD-10?

A) Different psychological disorders are distinct from each other
B) Diagnosis is possible only if the ultimate causes of symptoms are understood
C) Diagnosis of a specific disorder will ensure that a person will receive the most appropriate treatment
D) Two patients with the same diagnosis (e.g. 'Schizophrenia') are likely to be similar to each other.