5. Among the following propositions,
(1) $\sqrt{0.31}$ is in its simplest radical form;
(2) The final result of factoring $x^{3}+3 x^{2}-4$ is $(x+2)$
- $\left(x^{2}+x-2\right)$;
(3) If $a+b=1$, then $a^{3}+b^{3}+3 a b=1$;
(4) Rationalizing the denominator of $\frac{a b}{a \sqrt{b}+b \sqrt{a}}$ results in $\frac{a \sqrt{b}-b \sqrt{a}}{a-b}$

The number of correct propositions is ( ).
(A) 1
(B) 2
(C) 3
(D) 4
$5 . A$