Comparing the four proposed assignments directly by total cost

The assignment cost of assigning any one operator to any one machine is given in the table (rows U, V, W, X; columns I, II, III, IV): U$=(10,5,13,15)$, V$=(3,9,18,3)$, W$=(10,7,3,2)$, X$=(5,11,9,7)$. The optimal assignment is: