GWD中的一道老题，求NN解释

寻沫访华

Although improved efficiency in converting harvested trees into wood products may reduce harvest rates, it will stimulate demand by increasing supply and lowering prices, thereby boosting consumption.

A.in converting harvested trees into wood products may reduce harvestrates, it will stimulate demand by increasing supply and lowering prices, thereby boosting

B.in converting harvested trees into wood products may reduce harvestrates, demand will be stimulated because of increasing supply andlowering prices, which boost

C.of converting harvested trees into wood products may reduce harvest rates, it will stimulate demand by increasing supply and lowering prices,which boosts

D.of harvested trees being converted into wood products may reduceharvest rates, it will stimulate demand, because it will increase supplyand lower prices, thereby boosting

E.when harvested trees are converted into wood products may reduce harvest rates, demand will be stimulated because of increasing supplyand lowering prices, which boost

我记得语法原则中有写到 because of 后面必须要加名词的，加这种 doing形式是不对的，那这题为什么会选B，为什么A是不对的？

Passcode

我刚好也做了这道.我也选的A. 看这个LINK, M staff说答案不能是B
FYI－　

http://www.manhattangmat.com/forums/converting-harvested-trees-t10907.html