749. let f (r) be the function defined on the intervral (0,1) by if r is rational, -r otherwise. then f is continuous (a) at no point in (0, 1); (b) at exactly

749. Let f (r) be the function defined on the intervral (0,1) by if r is rational, -r otherwise. Then f is continuous (A) at no point in (0, 1); (B) at exactly one point in (0,1); (C) at more than one, but finitely many points in (0,1); (D) at infinitely many points in (0,1).