Which system becomes unstable due to uncontrolled sand mining in rivers?

Correct answer: River bed banks and aquatic habitat

Direct answer: A, the river bed, banks, and aquatic habitat become unstable. Uncontrolled sand mining removes sediment from a river system faster than natural processes can replace it. The reasoning is: sand supports the shape and level of the river bed; removing too much sand can deepen or unevenly lower the bed; banks may then lose support and erode; changing depth and flow can damage places where fish, plants, insects, and other organisms live. Therefore the connected river-bed, bank, and aquatic-habitat system is affected. Option A is correct because it names the physical and ecological parts directly disturbed by mining. Option B, Earth’s orbit, is unrelated to local removal of river sediment. Option C, the colour of a sea tide, is not the system that becomes unstable; mining may increase muddy water, but that is different from tide colour. Option D, celestial bodies, are not destabilised by river sand extraction. The effect can vary by amount and location, but uncontrolled mining creates a clear risk of erosion, habitat loss, and altered flow. Memory cue: “Remove river sand, weaken river shape and life.”