Plants A, B, C, and D Produce the Same Item

Plants A, B, C, and D produce the same item that is demanded by customers W, X, Y, and Z in the amounts shown 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.
What are the decision variables?
What is the objective function?
What are the constraints?
$$\begin{array} { | c | c | c | c | c | c | } \hline & \mathbf { W } & \mathbf { X } & \mathbf { Y } & \mathbf { Z } & \text { Total Supply } \\\hline \text { A } & \$ 3 & \$ 6 & \$ 4 & \$ 2 & 22,000 \\\hline \text { B } & \$ 7 & \$ 10 & \$ 8 & \$ 6 & 19,000 \\\hline \text { C } & \$ 10 & \$ 13 & \$ 11 & \$ 9 & 14,000 \\\hline \text { D } & \$ 12 & \$ 15 & \$ 13 & \$ 11 & 15,000 \\\hline \text { Total Demand } & 25,000 & 15,000 & 18,000 & 12,000 & \\\hline\end{array}$$