4. During the school holidays four building tasks, rebuilding a wall ( \(W\) ), repairing the roof ( \(R\) ), repainting the hall \(( H )\) and relaying the playground \(( P )\), need to be carried out at a Junior School. Four builders, \(A , B , C\) and \(D\) will be hired for these tasks. Each builder must be assigned to one task. Builder \(B\) is not able to rebuild the wall and therefore cannot be assigned to this task. The cost, in thousands of pounds, of using each builder for each task is given in the table below.

1. Use the Hungarian algorithm, reducing rows first, to obtain an allocation that minimises the total cost. State the allocation and its total cost. You must make your method clear and show the table after each stage.
2. State, with a reason, whether this allocation is unique.