题目选项分析

Scientists have found signs that moving water changed the chemical makeup of the surface of Mars in recent eras and have therefore concluded that the planet's crust harbors up to three times as much water as previously thought.

Scientists have found signs that moving water changed the chemical makeup of the surface of Mars in recent eras and they have concluded, therefore, that its crust harbors up to three times as much water, or more, than previously thought.