Which of the Following Are Required Conditions for the Distribution $\le$

Which of the following are required conditions for the distribution of a discrete random variable X that can assume values x_i?

A) 0 $\le$ p(x_i) $\le$ 1 for all x_i
B) $$\sum _ { \text {all } x _ { 1 } } p \left( x _ { i } \right) = 1$$
C) Both a and b are required conditions.
D) Neither a nor b are required conditions.