B. Paving the ground with three types of regular polygon tiles of equal side length, their vertices fit together perfectly without any gaps. If the number of sides of these three regular polygons are $x$, $y$, and $z$, then the value of $\frac{1}{x}+\frac{1}{y}+\frac{1}{z}$ is ( ).
(A) $\frac{1}{3}$
(B) $\frac{1}{2}$
(C) $\frac{2}{3}$
(D) 1
B. B.

The interior angles of these three regular polygons are respectively equal to
$$
\frac{x-2}{x} \times 180^{\circ}, \frac{y-2}{y} \times 180^{\circ}, \frac{z-2}{z} \times 180^{\circ} \text{. }
$$

Since the three tiles fit together perfectly without gaps, the sum of these three angles is $360^{\circ}$.
$$
\begin{array}{l}
\text { Therefore, } \frac{x-2}{x}+\frac{y-2}{y}+\frac{z-2}{z}=2 \\
\Rightarrow \frac{1}{x}+\frac{1}{y}+\frac{1}{z}=\frac{1}{2} .
\end{array}
$$