question: What is the problem with CSI (channel state information) acquisition in RIS (Reflective Intelligent Surface)-aided systems? option 1: The RIS elements have limited capability of transmitting and receiving RF signals. option 2: The user-RIS and RIS-BS channel links cannot be estimated using conventional pilot-assisted channel estimation methods. option 3: The size of a RIS is usually very large, imposing difficulty on the channel estimation problem. option 4: The cascaded channel estimation problem is a linear estimation problem. option 5: The RIS-BS channel varies much slower than the user-BS and user-RIS channels. 