[Solution] pOH = 5, $[OH^-] = 10^{-5}$, $[Ca^{2+}] = 0.5 \times 10^{-5}$; $K_{sp} = 0.5 \times 10^{-5} \times (10^{-5})^2 = 0.5 \times 10^{-15}$. Answer (1).
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