While they remove caron 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...........
B. pharmaceuticals, and sources of building material and firewood, are providers of
C. pharmaceuticals and, sources of building material and forewood that are providing
D. pharmaceuticals and sources of building material and firewood, for providing
E. pharmaceuticals and , as sources of building material and firewood, provide
参考答案是E，说supply ... and ... provide是平行结构。
但个人认为是D，pharmaceuticals and sources才是平行结构
E：典型在and连接的两动词之间嵌入“插入语” ，GMAT中很多的
D应该把for去掉
类似的如which：
It has more than 60 specialized subject libraries as well as the university library,which,as a copyright library,is entitled to a copy of every book published in Britain.
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