$4.(10$ points$)$ Solve the following transportation problem: warehouses A$,$ B$,$ and C have $40,30,$ and $50$ units respectively. Stores $1,2,$ and $3$ need $50,10,$ and $40$ units respectively $($you may introduce a dummy store to absorb the excess supply $-$ the cost to reach it should be zero$).$ The costs of the routes from each warehouse to each store $($per unit$)$ are summarized below: