According to I.S. : 456, slabs which span in two directions with corners held down, are assumed to be divided in each direction into middle strips and edge strips such that the width of the middle strip, is______________?

A. Half of the width of the slab
B. Two -third of the width of the slab
C. Three-fourth of the width of the slab
D. Four-fifth of the width of the slab