题目选项分析

The Grand Canyon,

previously thought to have been carved out by the Colorado River, was eroded away by a lava flow, according to a newer theory, and

took on its current form at an earlier date 6 to 7 million years ago.

The Grand Canyon,

previously thought to have been carved out by the Colorado River, according to a newer theory, it eroded away by a lava flow and

took on its current form at an earlier date 6 to 7 million years ago.

题目基本信息

难度：难