After testing each unused cell by the stepping$-$stone method in the transportation problem and finding two cells with a negative improvement index

$$ you should check to be sure that you don't have to add a dummy source or dummy destination.

you would make an improvement based on one of the two negative improvement indices and then re$-$compute an improvement index for each empty cell.

$$ you could stop because you already had the optimal solution.

$$ you would make two improvements and then re$-$compute an improvement index for each empty cell.

$$ you could stop because there was only one cell with a negative improvement index.