Can somebody explain the concept behind this , I did not fully understand the answer.

question

The project we are considering is the building of a road. About half the road workers are unemployed before the project and half are employed. The current wage rate is $15.fhour but unemployed workers would be willing to work for $lfhour. The project needs 100D workers. In cost benet analysis using the wage rate. the accounting cost for a unit of labor is $1530 In cost benet analysis using the wage rate. the economic cost for a unit of labor includes the lower reservation wage of unemployed workers 5 In cost benet analysis using the wage rate. the accounting cost for a unit of labor is the same as the economic cost. The total economic cost of labor for this project is $15.0 Aand B