请教一道prep...

64.
            (27882-!-item-!-188;#058&003676)

The majority of scientists believe that to reduce and stabilize atmospheric concentrations of carbon dioxide, emissions must be cut at the source by fossil fuels that are burned more efficiently and, in some cases, by alternatives such as hydrogen fuel cells that can replace fossil fuels altogether.

(A) fossil fuels that are burned more efficiently and, in some cases, by alternatives such as hydrogen fuel cells that can replace fossil fuels altogether

(B) fossil fuels that are burned more efficiently and, in some cases, those that are replaced altogether with alternatives such as hydrogen fuel cells

(C) burning fossil fuels more efficiently and, in some cases, by replacing fossil fuels altogether with alternatives such as hydrogen fuel cells

(D) more efficiency in the burning of fossil fuels and, in some cases, by alternatives such as hydrogen fuel cells that can replace fossil fuels altogether

(E) the more efficient burning of fossil fuels and, in some cases, by fossil fuels that are replaced altogether with alternatives such as hydrogen fuel cells

答案是C，A为什么不对啊？by后面只能加分词不能加名词么？

体会一下意思：

emissions must be cut at the source 是由于 fossile fuels这个东西还是 burning fossil fuel这个动作？

PS： by doing  是 GMAT优选结构