Which one of the following charts represents a probability distribution : a) X P(x) 0 -0.1 1 0.1 2 0.4 3 0.6 b) X P(x) 0 0.3 1 0.2 2 0.1 3 0.35 c) X P(x) 0 0.1 1 0.2 2 0.4 3 0.1 d) X P(x) 0 0 1 0 2 1 3 0