Correct Option: B. The volume of water has been increased by rainfall Explanation of the Correct Answer: Why Option B is Correct: - Rivers overflow their banks primarily due to an increase in the volume of water flowing through them. This increase can be caused by heavy rainfall, which leads to more water entering the river system than it can handle. When the volume of water exceeds the river's capacity, it spills over the banks, resulting in flooding. - Rainfall can lead to surface runoff, where water flows over the ground and directly into the river, increasing its flow rate and volume. Additionally, if the ground is saturated from previous rainfall, it cannot absorb more water, leading to increased runoff into the river. Why the Other Options are Incorrect: Option A: Its valley has become shallow due to the deposition of alluvium - While it is true that alluvial deposition can affect river dynamics, a shallow valley does not directly cause overflow. In fact, a shallow valley may reduce the likelihood of overflow because it can hold more water before spilling over. The primary cause of overflow is the volume of water, not the depth of the valley. Option C: Farmers block its channel with farm wastes - Although blocking a river's channel can contribute to localized flooding, it is not a primary cause of river overflow. The main issue with blockages is that they can create temporary backups, but they do not account for the overall increase in water volume that leads to significant overflow events. Therefore, this option is weaker compared to the direct impact of increased rainfall. Option D: Its valley is narrow - A narrow valley can indeed lead to higher water levels during flooding events, but it does not inherently cause overflow. A narrow valley may actually exacerbate flooding if the volume of water is high, but it is not the root cause. The overflow is still fundamentally linked to the volume of water, which is not addressed by the valley's width alone. Summary of Key Points:

• Rivers overflow primarily due to increased water volume from rainfall.
• Heavy rainfall leads to surface runoff, increasing river flow and potential overflow.
• Other factors like valley shape and blockages can influence flooding but are not primary causes.
• Understanding the relationship between rainfall and river capacity is crucial for flood management.

This detailed explanation should help clarify why option B is the correct answer and provide a deeper understanding of river overflow dynamics.