12. 承第 11 題，當粒子以穩定定速下降且速度以 \(v_t\) 代表，則該拖曳係數 (drag coefficient) 爲：
(A) \(\frac{3}{4} \frac{D(\rho_s - \rho_f)g}{\rho_f v_t^2}\)
(B) \(\frac{3}{4} \frac{D^2(\rho_s - \rho_f)g}{\rho_f v_t^2}\)
(C) \(\frac{4}{3} \frac{D(\rho_s - \rho_f)g}{\rho_f v_t^2}\)
(D) \(\frac{4}{3} \frac{D^2(\rho_s - \rho_f)g}{\rho_f v_t^2}\)

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