For each of the following functions, state whether it is a valid cumulative distribu- tion function. Justify your answers. 0 (b) (d) 0 0 if < 1 if x > I if x < 0 if 0 < x < I if a: < 0 For Q > o, O: 0 if a: > 0 if a: < 0 if k < x < k -F 1, for k t!