If the ratio of long and short spans of a two way slab with corners held down is r, the actual reduction of B.M. is given by

A. (5/6) (r/1 + r²) M
B. (5/6) (r²/1 + r²) M
C. (5/6) (r²/1 + r3) M
D. (5/6) (r²/1 + r4) M