请教2道语法题，GWD题

1.      Most of Portugal’s 250,000 university students boycotted classes in a one-day strike to protest a law that requires them to contribute $330 a year toward the cost of higher education, previously paying $7 per year.
        
A)        year toward the cost of higher education, previously paying $7 per year   
B)        year toward the cost of higher education, for which was previously paid $7 per year   
C)        year, compared to the previously $7 per year, toward the cost of higher education

D)        year toward the cost of higher education, instead of the $7 per year required previously   
E)        year as opposed to the $7 per year required previously for the cost of higher education

答案是D，大概是上过新东方的缘故，我做题只要看到instead of 就直接排除了，都说not but 优于rather than 优于instead of，一般instead of 都错，所以这题就弄不懂了，能解释下B为什么错吗。。。难道是for表原因？？我觉得这里可以是paid for，将for提前呀

2. 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.

A. to be huge reservoirs of energy, possibly causing sea floor instability, and
B. to be huge reservoirs of energy, possibly causing sea floor instability, and even as
C. as huge reservoirs of energy, possibly causing sea floor instability, and
D. as huge reservoirs of energy, and the possible cause of sea floor instability,
E. as huge reservoirs of energy, as possible causes of sea floor instability, and even as

答案是 A

这题我就想知道下to be 和 as 在这里的用法有什么不同？为什么一定是to be，不是as

谢谢

1。A选项，用现代分词paying做状语，表伴随结果或伴随动作，没有道理，排除。

   B选项，which就近修饰higher education，逻辑不通，应是pay for cost。

   C选项，compare to 不同于compare with

   E选项，oppose to 表示反对，不妥；require for 永远错。

2.to be 的意思是“还没有成为，即将成为”

   as 的意思是“已经成为”。

很显然，potential to be正确。