= Solution
The strange quark is light enough compared with typical hadronic scales to permit an approximate
$$
SU(3)_L\times SU(3)_R\longrightarrow SU(3)_V
$$
chiral expansion. It adds kaons and the eta to the pseudo-Goldstone multiplet, although convergence is poorer because $m_s$ is substantially larger than $m_u,m_d$.
The charm quark is too heavy for the same light-flavour chiral expansion. At low energies it is integrated out; processes containing charm are instead treated using heavy-quark expansions or other effective theories. Thus the analysis extends approximately to $s$, but not to $c$ as another pseudo-Goldstone-producing light flavour.
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