Given the system logs in the table below that shows the sequence of operations of four transactions T1,T2,T3 and T4. a) What is the purpose of using checkpoint? b) In case of recovery using deferred update, which transactions will be undone? which transactions will be redone? which transactions will be ignored? In your answer, you need to discuss the status of all transactions T1,T2,T3 and T4 and to explain why