13 土壤濾層（Filter）設計選擇適當土壤材料作為濾層材使被保護土壤（Protected soil）不會被淘空。下列何者符合濾層土壤的條件？
(A) D85, filter / D85, protected soil ≤ 4～5
(B) D15, filter / D85, protected soil≤ 4～5
(C) D15, filter / D85, protected soil ≤ 4～5
(D) D15, filter / D15, protected soil ≤ 4～5