Which of the following statement is/ are correct about Alzheimer’s disease?

• A. Alzheimer disease is a degenerative disease of the brain that causes dementia and it has been found that it is closely linked with type 2 diabetes.
• B. Alzheimer's disease is not a mental illness but it can cause symptoms related to mental health like depression, anxiety etc.
• C. Alzheimer's disease is categorised into mild Alzheimer's disease, moderate Alzheimer's disease and severe Alzheimer's disease.
• D. All the above are correct