题目选项分析

Geologists have found that streams in the Karoo basin of South Africa changed suddenly at the end of the Permian period 250 million years ago, from the meandering shape typical in well-vegetated zones to the braided pattern found in areas where it is without deep-rooted vegetation for holding the soil together.

Geologists have found that streams in the Karoo basin of South Africa changed suddenly at the end of the Permian period 250 million years ago, from the meandering shape typically found in well-vegetated zones to the braided pattern found in areas without deep-rooted vegetation to hold the soil together.