f the straight lines , ax + amy + 1 = 0 , bx + (m + 1) by + 1 = 0 and cx + (m + 2)cy + 1 = 0, m ≠ 0 are concurrent then a, b, c are in :
(A) A.P. only for m = 1
(B) A.P. for all m
(C) G.P. for all m
(D) H.P. for all m.