Consider a RAID$-4$ disk configuration involving five disks A$,$ B$,$ C$,$ D and E where disk E stores the parity. Examine the following six scenarios involving read and write operations in the presence of disk failures in this configuration:

All disks are up$.$ There is a write operation on block $10$ of disk C$.$

Disk B is down and all other disks are up$.$ There is a read operation on block $20$ of disk C$.$

Disk C is down and all other disks are up$.$ There is a read operation on block $30$ of disk C$.$

Disk D is down and all other disks are up$.$ There is a write operation on block $40$ of disk C$.$

Disk E is down and all other disks are up$.$ There is a read operation on block $50$ of disk C$.$

Disk E is down and all other disks are up$.$ There is a write operation on block $60$ of disk C$.$

In each of the above scenarios determine how many disk accesses happen in order to process the I$/$O operation. Assume that the goal is to allow as many of the I$/$O operations as possible, in the presence of disk failures. When counting disk accesses do not count accesses to disks that are down.

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$$a$)$The number of disk accesses for the second scenario is $2.$b$)$The number of disk accesses for the third scenario is $4.$c$)$The number of disk accesses for the third scenario is $1.$d$)$The number of disk accesses for the sixth scenario is $3.$