题目选项分析

While they remove carbon dioxide from the air, conserve soil and water, and house thousands of species, forests also supply potentially valuable pharmaceuticals and, as sources of building material and firewood, provide employment for millions worldwide.

While they remove carbon dioxide from the air, conserve soil and water, and house thousands of species, forests also supply potentially valuable pharmaceuticals, and sources of building material and firewood, are providers of employment for millions worldwide.