Fill in the missing event that causes livelock.At an insurance company, the system performs many daily transactions. One day, the following series of events ties up the system:

1. Process P1 wishes to show a payment, so it issues a command to read the balance, which is stored in cylinder 20 of a disk pack.

2. ____

3. P2 gains control of the I/O channel and issues a command to write someone else's payment to a record stored in cylinder 310. If the command is not "locked out," P2 will be put on hold while the control unit moves the arm to cylinder 310.

4. Because P2 is "on hold," the channel is free to be captured again by P1, which reconfirms its command to "read from cylinder 20."

5. Since the last command from P2 had forced the arm mechanism to cylinder 310, the disk control unit begins to reposition the arm to cylinder 20 to satisfy P1. The I/O channel would be released because P1 is once again put on hold, so it could be captured by P2, which issues a WRITE command only to discover that the arm mechanism needs to be repositioned.

A. While the control unit is moving the arm to cylinder 20, P1 is put on hold and the I/O channel is free to process the next I/O request.
B. P1 discovers that another process has locked the portion of the disk it needs to access.
C. P2 is put on hold while the control unit moves the arm to satisfy P1's request
D. P1 is unable to find the information it needs, so requests a different READ operation for a different cylinder.

Answer: A