8. If one edge length of a tetrahedron is $x$, and the other edges are all 1, the volume is $F(x)$, then the function $F(x)$ on its domain
(A) is an increasing function but has no maximum value;
(B) is an increasing function and has a maximum value;
(C) is not an increasing function and has no maximum value;
(D) is not an increasing function but has a maximum value.
8. (D)

In the tetrahedron $ABCD$,
let $AB = x$,
$$
AC = AD = BC = BD = CD = 1,
$$

Fix $\triangle BCD$ as the base. Its three sides are all 1, and the area is $\frac{\sqrt{3}}{4}$, which is a constant. As $\triangle ACD$ rotates around $CD$, the value of $x$ changes, and the height from $A$ to the base, $AO$, also changes. This indicates that the domain of $F(x)$ is $(0, \sqrt{3})$.

Let $AO = h$. The volume of the tetrahedron, when $S_{\triangle BCD} = \frac{\sqrt{3}}{4}$, depends on the changes in $h$.
When $x \rightarrow 0$ or $x \rightarrow \sqrt{3}$,
$$
F(x) \rightarrow 0,
$$

Therefore, $F(x)$ is not an increasing function.
When the plane $ACD \perp$ plane $BCD$, $h$ reaches its maximum value, at which point $F(x)$ reaches its maximum value.
Thus, $F(x)$ is not an increasing function but has a maximum.