State Whether or Not Each of the Following Are Valid $\begin{array}{c|cccc}x & 0 & 1 & 2 & 3 \\\hline p(x) & .15 & .25 & .35 & .45\end{array}$

State whether or not each of the following are valid probability distributions, and if not, explain why not. a.
$\begin{array}{c|cccc}x & 0 & 1 & 2 & 3 \\\hline p(x) & .15 & .25 & .35 & .45\end{array}$
b.
$\begin{array}{c|cccc}x & 2 & 3 & 4 & 5 \\\hline p(x) & -.10 & .40 & .50 & .25\end{array}$
C.
$\begin{array}{c|ccccc}x & -2 & -1 & 0 & 1 & 2 \\\hline p(x) & .10 & .20 & .40 & .20 & .10\end{array}$