Q6:After more than four decades of research and development, a new type of jet engine is being tested that could eventually propel aircraft anywhere in the world within two hours or help boost cargoes into space at significantly lower costs than current methods permit.
A. tested that could eventually propel aircraft anywhere in the world within two hours or help B. tested that could eventually have the capability of propelling aircraft anywhere in the world within two hours or to help C. tested, eventually able to propel aircraft anywhere in the world within two hours, or helping D. tested, and it eventually could propel aircraft anywhere in the world within two hours or helping E. tested, and it could eventually have the capability to propel aircraft anywhere in the world within two hours or help
我选了A，答案给了E。请教A怎么错了呢？E中could have the capability是不是有点redundant呢？另外请教一个小语法，capable的用法是is capable of doing...，have the capability后面是to do和of doing都可以的吗？我好像见到过两种用法，但是不确定。 | 
