Plants A$,$ B$,$ C$,$ and D produce the same item that is demanded by customers W$,$ X$,$ Y$,$ and Z$.$ Customer demand and plant capacity are shown in the bottom row and rightmost column in the table. The cost to ship one unit between each combination of plants and customers is also shown in the table. The corporation wishes to minimize the total transportation costs of this assignment problem. Use the table to answer the following question$($s$).$

$\backslash$table$[[,$W$,$X$,$Y$,$Z$,$Total Supply$],[$A$,$$$3,$$$6,$$$4,$$$2,22,000],[$B$,$$$7,$$$10,$$$8,$$$6,19,000],[$C$,$$$10,$$$13,$$$11,$$$9,14,000],[$D$,$$$12,$$$15,$$$13,$$$11,15,000],[$Total Demand,$25,000,15,000,18,000,12,000,]]$

What is a suitable constraint?