题目选项分析

Prospecting for gold during the California gold rush was a relatively easy task, owing to erosion, prehistoric glacier movement, and volcanic activity that had thrust ancient, gold-bearing riverbeds to the surface, and putting gold literally within reach of anybody with a pan or shovel.

Prospecting for gold during the California gold rush was a relatively easy task, since erosion, prehistoric glacier movement, and ancient, gold-bearing riverbeds thrust to the surface by volcanic activity put gold literally within reach of anybody with a pan or shovel.