5. Given that $I$ and $O$ are the incenter and circumcenter of $\triangle ABC$ respectively, and points $B, C, I, O$ are concyclic. Then the degree measure of $\angle BAC$ is ( ).
(A) $30^{\circ}$
(B) $45^{\circ}$
(C) $60^{\circ}$
(D) $75^{\circ}$
5. C.

As shown in Figure 5, connect $I B$.
$$
I C, O B, O C \text {. }
$$

From 1.0, the incenter and circumcenter of $\triangle A B C$ respectively give
$$
\begin{array}{c}
\angle B I C \\
=90^{\circ}+\frac{1}{2} \angle B A C, \\
\angle B O C=2 \angle B A C .
\end{array}
$$

Since $B, C, I, O$ are concyclic,
$$
\angle B O C=\angle B I C \text {. }
$$

Thus, $2 \angle B A C=90^{\circ}+\frac{1}{2} \angle B A C$
$$
\Rightarrow \angle B A C=60^{\circ} \text {. }
$$