How are rivers useful for agriculture?

Correct answer: They provide water for irrigation

Agriculture depends on a reliable supply of water because crops need water for germination, growth, nutrient transport, and development. Rivers provide surface water that can be directed through canals, pipes, pumps, or other irrigation systems to fields. This is especially useful when rainfall is irregular or insufficient. River water must still be managed carefully because excessive diversion can harm downstream users and river ecosystems.

Option B is correct because irrigation means supplying water to cultivated land, and rivers are an important source for it. Option A is scientifically opposite to the usual role of river water, while C and D describe harm rather than agricultural benefit. The question asks how rivers help farming, so the direct connection is the provision of irrigation water. That makes B the appropriate choice.