ANS:C - both (a) and (b)

1. Drains with beds of rocks and gravels:
   • Drains with beds of rocks and gravels typically have natural or unmodified channels where the bottom and sides consist of loose rocks, gravel, or coarse sediments.
   • These drains may be found in natural watercourses, agricultural ditches, or areas where water runoff needs to be managed but where formalized drainage structures are not constructed.
   • The presence of rocks and gravels helps in providing some degree of stability to the drain bed and allows for water infiltration and percolation into the soil.
   • However, drains with beds of rocks and gravels are susceptible to erosion and may require maintenance to prevent sediment accumulation and blockages.
2. Drains lined with stones:
   • Drains lined with stones feature a constructed channel with defined boundaries and a structured lining made of stones or concrete blocks along the bottom and sides.
   • The purpose of lining drains with stones is to stabilize the drain bed and prevent erosion of the soil or sediment beneath.
   • Lining with stones helps to maintain the shape and integrity of the drain, even during periods of high flow or heavy rainfall.
   • The stones used for lining can vary in size and composition depending on the drainage requirements and the surrounding environment.
   • Lined drains are commonly used in urban drainage systems, roadside ditches, and engineered drainage channels where the control of water flow and prevention of erosion are critical.