GWD 29-41

Gas hydrates, chemical compounds of water and natural gas, are increasingly being studied for their potential to be huge reservoirs of energy, possibly causing sea floor instability, and significant contributors to global warming.

1. to be huge reservoirs of energy, possibly causing sea floor instability, and
2. to be huge reservoirs of energy, possibly causing sea floor instability, and even as
3. as huge reservoirs of energy, possibly causing sea floor instability, and
4. as huge reservoirs of energy, and the possible cause of sea floor instability,
5. as huge reservoirs of energy, as possible causes of sea floor instability, and even as

答案给的是D  

但我怎么看都不像，我选E   根据as A ,as B and as C这样的平行结构~

请NN们给个说法～～

这题ANS本来就是E啊?只看过A, B, and C

还没看过D选A, and B, C根本完全不平行!!