5. In a sports meet, ten athletes participate in a table tennis competition, where each pair of athletes plays exactly one match. During the competition, the first athlete wins $x_{1}$ games and loses $y_{1}$ games; the second athlete wins $x_{2}$ games and loses $y_{2}$ games; $\cdots \cdots$ the tenth athlete wins $x_{10}$ games and loses $y_{10}$ games. Let
$$
\begin{array}{l}
M=x_{1}^{2}+x_{2}^{2}+\cdots+x_{10}^{2} . \\
N=y_{1}^{2}+y_{2}^{2}+\cdots+y_{10}^{2} .
\end{array}
$$

Then ( ).
(A) $M < N$
(B) $M > N$
(C) $M = N$
(D) The relationship between $M$ and $N$ is uncertain