5. If $a, b, c \in \mathbf{N}$, and
$$
29 a+30 b+31 c=366 \text{. }
$$

then $a+b+c=(\quad)$.
(A) 10
(B) 12
(C) 14
(D) 16
5.B.

From $29a + 30b + 31c = 366 (a, b, c \in \mathbf{N})$, we get $29(a + b + c) \leqslant 366 \leqslant 31(a + b + c)$.
Therefore, $11 < a + b + c < 13$.