题目选项分析

After determining the levels of calcium carbonate and the types of fossilized algae that had been present in core samples taken from the bottom of Lake Titicaca in the Andes, that made it possible for a group of scientists to reconstruct the history of precipitation in tropical South America over the past 25,000 years.

After determining the levels of calcium carbonate and the types of fossilized algae present in core samples taken from the bottom of Lake Titicaca in the Andes, a group of scientists was able to reconstruct the history of precipitation in tropical South America over the past 25,000 years.