Identify the type of random variable-binomial, Poisson, or hypergeometric-described by each of the following probability distributions:
$$
\begin{aligned}
\text { sa. } p(x) &=\frac{.5^{x} e^{-5}}{x !}(x=0,1,2, \ldots) \\
\text { b. } p(x) &=\left(\begin{array}{c}
6 \\
x
\end{array}\right)(.2)^{x}(.8)^{6-x} \quad(x=0,1,2, \ldots, 6) \\
\text { c. } p(x) &=\frac{10 !}{x !(10-x) !}(.9)^{x}(.1)^{10-x} \\
(x&=0,1,2, \ldots, 10)
\end{aligned}
$$