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\frac{\mathrm{dh}}{\mathrm{dt}}=\frac{\mathrm{dv} / \mathrm{dt}}{\pi \mathrm{r}^{2}}=\frac{\mathrm{K} \pi \mathrm{r}^{2}}{\pi \mathrm{r}^{2}}=\mathrm{K} \quad