Transportation doubt in Cost Management

I need some help realting to an exam problem in

Transportation - Cost management .

May 2007 .

The problem is as below :

            8                      6          4

11           2       8          6          2          /           18

 10

9          9          12        9          6          /           10

                        8

7          6          3          7          7          /           8

 2                                 2

9          3          5          6          11        /           4

12        8          8          8          4         

In this

when we solve we get degeneracy , No. of allocations not equal to m+n-1

and we need to allocate 'e' to the least cost independent cell in the unalloted regular columns

Here least cost is 3 as seen above (in the fourth row second coloumn)

but in the problem solved

They  said that the cell is not independent we allot to next least cost independent cell 5 next right to 3

WHAT DOES INDEPENDENCE HERE MEANS?