C because graduated terrace steps are commonly used to farm on hilly or mountainous terrain. Terraced fields decrease both erosion and surface runoff, and may be used to support growing crops that require irrigation, such as rice. ( It basically means that terraced farming is farming over steps and using rain to wash away old soil so that's where the wet part comes in)
As the debris is geologically event with the water-laden masses of soils and rocks fragments and fall down from the mountains and a funnel into the stream channels and also, objects there paths with thick mud and blocks valleys. The volume if the debris flows generally ranged from the 100,000 cubic meters and frequent n the mountainous regions and can be very destructive.
