Which of the six underlying control conceptsisolation, redundancy, comparison, oversight, accountability, or assistanceis involved in the following control activities? (Note: some controls may involve more than one concept.) a. Locking the warehouse door b. Personnel training c. Standard cost variance analysis d. Having security guards patrol a plant e. Segregating the functions of approving purchase requisitions and preparing purchase orders f. Storing backup copies of files at a remote location g. Checking account numbers against a reference file of authorized numbers h. Transmitting an acknowledgment signal following a telecommunications message i. Adding a column of figures two times to see if you got the right answer j. Having a supervisor check a calculation k. Recording an operators ID number in a stored transaction record l. Granting a user a read-only right to a file