To generate the j the column of the flexibility matrix________________?

A. a unit force is applied at coordinate j and the displacements are calculated at all coordinates
B. a unit displacement is applied at co-ordinate j and the forces are calculated at all coordinates
C. a unit force is applied at coordinate j and the forces are calculated at all coordinates
D. a unit displacement is applied at co-ordinate j and the displacements are calculated at all co-ordinates