The neutral kaon $K^{0}$ is composed of a down quark (d) and an anti-strange quark ($\bar{s}$). The charged pions $\pi^{+}$ and $\pi^{-}$ are composed of an anti-down and an up quark ($\bar{d}$, u) and a down and an anti-up quark (d, $\bar{u}$) respectively.
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