Which three statements are true about memory buffer allocation by a MySQL Server?（）

• A、Global buffers such as the InnoDB buffer pool are allocated after the server starts, and are never freed
• B、Thread buffers are allocated when a client connects, and are freed when the client disconnects
• C、Buffers that are needed for certain operation are allocated when the operation starts, and freed when it ends
• D、User buffers are allocated at server startup and freed when the user is dropped
• E、All dynamic buffers that are set with a SET GLOBAL statement immediately get allocated globally, and are never freed