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, that provide employment for millions worldwide. A. pharmaceuticals, and, as sources of building material and firewood, that provide B. pharmaceuticals, and sources of building material and firewood, are providers of C. pharmaceuticals and, as sources of building material and firewood that are provide D. pharmaceuticals and as sources of building material and firewood, for providing E. pharmaceuticals and, as sources of building material and firewood, provide 这个答案选E ，看不懂啊，望指点下! | 
