For each of the following, determine whether the given values can serve as the values of a probability distribution of a random variable with the range $x=1,2,3$, and 4:
(a) $f(1)=0.25, f(2)=0.75, f(3)=0.25$, and $f(4)=$
$-0.25$
(b) $f(1)=0.15, f(2)=0.27, f(3)=0.29$, and $f(4)=0.29 ;$
(c) $f(1)=\frac{1}{19}, f(2)=\frac{10}{19}, f(3)=\frac{2}{19}$, and $f(4)=\frac{5}{19}$.