A river training work is generally required when the river is A. Meandering B. Aggrading C. Degrading D. All the above

The correct answer is: D. All the above

A river training work is generally required when the river is meandering, aggrading, or degrading.

• Meandering is a natural process that occurs when a river flows over a relatively flat surface. The river will tend to follow the path of least resistance, which often results in a winding course. Meandering rivers can cause problems for humans, as they can flood and erode land. River training works can be used to straighten or stabilize meandering rivers.
• Aggrading is a process that occurs when a river deposits sediment. This can happen when the river slows down, such as when it enters a lake or estuary. Aggrading rivers can cause problems for humans, as they can fill in channels and create floodplains. River training works can be used to remove sediment from aggrading rivers.
• Degrading is a process that occurs when a river erodes its bed. This can happen when the river flows over a hard surface, such as bedrock. Degrading rivers can cause problems for humans, as they can undercut banks and cause landslides. River training works can be used to protect banks from erosion.

In conclusion, a river training work is generally required when the river is meandering, aggrading, or degrading. These works can be used to straighten or stabilize meandering rivers, remove sediment from aggrading rivers, or protect banks from erosion.