4. The amount of sediment carried by a river accurately indicates the amount of soil eroding from that river's drainage basin. Fact 1: The Huang carries much more sediment annually than the Ganges. Conclusion: The rate of erosion per unit surface area is greater in the Huang's basin than in the Ganges'. Which additional fact, if true, allows the conclusion to be properly drawn?
Answer: D
The rate of erosion per unit surface area is greater in the Huang's basin than in the Ganges'.
If the Huang's drainage basin covers a smaller surface area than the Ganges', then the amount of sediment it carries annually would indicate a higher rate of erosion per unit area compared to the Ganges, supporting the conclusion drawn.
A) Certain soil-conservation measures successful in the Huang basin have never been tried in the Ganges basin.
This option does not provide relevant information regarding the surface area of the drainage basins or their respective sediment transport. While it might suggest differences in management practices, it does not directly support the conclusion about erosion rates per unit area.
B) Many farming practices improve short-term productivity but increase erosion.
While this statement might imply factors that contribute to erosion, it does not address the comparative sizes of the Huang and Ganges basins. Thus, it fails to provide the necessary context needed to validate the conclusion regarding erosion rates per unit surface area.
C) More logging has recently been done along the Huang.
This choice suggests a potential increase in erosion due to logging activities, but it does not clarify the relationship between the sizes of the basins or provide a basis for comparing erosion rates per unit surface area. Therefore, it does not support the conclusion effectively.
D) The Huang's drainage basin covers a smaller surface area than the Ganges'.
This fact directly supports the conclusion. If the Huang has a smaller drainage area but carries more sediment, it logically follows that the erosion rate per unit area must be greater than that of the Ganges, thus validating the conclusion.
Conclusion
The correct answer is D, as it provides the necessary context to understand the relationship between the sediment carried by the rivers and the surface area of their respective drainage basins. Options A, B, and C lack the pertinent information needed to substantiate the conclusion regarding erosion rates, making them insufficient in this context.