How likely is it NOW that a comet/asteroid WILL STRIKE Earth?
C mentions "how likely a comet/asteroid will be". Nope. The likelihood is something that exists now, not in the future. (That's the definition of "likelihood": How probable does this event seem right now? There's no "future likelihood" here. In the future, the event either happens or doesn't happen.)
D mentions the likelihood (as measured at present) that a comet/asteroid will collide (in the future) with Earth. That makes sense.

明天有50%可能下雨（也有50%可能不下雨）。这个有可能是基于今天的情报预测明天，从今天来看，存在明天下雨的可能性也存在明天不下雨的可能性，换句话说可能性是今天的事，下雨才是明天的事；而假如真正到了明天，下雨或者不下雨都会成为既定事实，下了雨就是100%下雨，没下雨就是100%不下雨，并不存在所谓的下雨可能性和不下雨的可能性的概念，所以逻辑上不存在“将会可能发生什么”，只存在“（现在来看）可能将会发生什么”

From Manhattan/KMF

没有 future likely。 sb is likely to xx。而不是 sb will be likely to do

likelihood that sb will 。。。 makes sense

Manhatten 的 staff如此说：
for those of you who are not native speakers of english - the best approach to problems such as this one is:
* note the differences in usage between the formal and informal
- e.g., "rate how likely" vs. "rate the likelihood that..."
* remember what these differences look like, so that you can make similar distinctions in the future.

注重中心词：E中likelihood的修饰部分，即，a particular asteroid or comet that may collide with Earth，的核心词为“一个特定的行星或彗星（a particular asteroid or comet）”。在逻辑上，应是“特定行星撞击的可能性”，而不是“特定行星的可能性”。因此，likelihood的修饰部分应用同位语从句。
而rate对应的中心词应该只有likelihood才对，但在逻辑上，真正被“打分”的对象应该是小行星撞击地球的“可能性”而不是“撞击”这个事件本身（打出来的分，肯定是某一个或几个小行星撞击的百分比嘛）。因此，rate宾语的核心词应是likelihood。

完全没有被这个解析说服....RON的答案也有点偏native,感觉有点难理解

1. rate的核心词是likelihood，而不是collide
2. develop a scale对于rate这个动作发生的状态不一定产生改变，这里for doing表示用途，只是为了rate而develop，但不会develop之后马上rate完

c两个点错了：
1.to rate 改为for rating
！！2.当rate宾语从句时，宾语从句中的核心词是collide，而真正应该做rate宾语的是likelihood，

C: astronomers have developed a scale to rate how likely a particular asteroid or comet [will be to]

D: astronomers have developed a scale for rating the likelihood [that] a particular asteroid or comet will

E: astronomers have developed a scale that rates the likelihood [of] a particular asteroid or comet that may

How likely is it NOW that a comet/asteroid WILL STRIKE Earth?
C mentions "how likely a comet/asteroid will be". Nope. The likelihood is something that exists now, not in the future. (That's the definition of "likelihood": How probable does this event seem right now? There's no "future likelihood" here. In the future, the event either happens or doesn't happen.)
D mentions the likelihood (as measured at present) that a comet/asteroid will collide (in the future) with Earth. That makes sense.

明天有50%可能下雨（也有50%可能不下雨）。这个有可能是基于今天的情报预测明天，从今天来看，存在明天下雨的可能性也存在明天不下雨的可能性，换句话说可能性是今天的事，下雨才是明天的事；而假如真正到了明天，下雨或者不下雨都会成为既定事实，下了雨就是100%下雨，没下雨就是100%不下雨，并不存在所谓的下雨可能性和不下雨的可能性的概念，所以逻辑上不存在“将会可能发生什么”，只存在“（现在来看）可能将会发生什么”

for those of you who are not native speakers of english - the best approach to problems such as this one is:
* note the differences in usage between the formal and informal
- e.g., "rate how likely" vs. "rate the likelihood that..."
* remember what these differences look like, so that you can make similar distinctions in the future.

选项E：likelihood的修饰部分，即，a particular asteroid or comet that may collide with Earth，的核心词为“一个特定的行星或彗星（a particular asteroid or comet）”。在逻辑上，应是“特定行星撞击的可能性”，而不是“特定行星的可能性”。因此，【likelihood的修饰部分应用同位语从句！】。

RON:

This distinction comes into play when you look at the verbs in C and D. Different verbs.

C mentions "how likely a comet/asteroid will be". Nope. The likelihood is something that exists now, not in the future. (That's the definition of "likelihood": How probable does this event seem right now?
There's no "future likelihood" here. In the future, the event either happens or doesn't happen.)

D mentions the likelihood (as measured at present) that a comet/asteroid will collide (in the future) with Earth. That makes sense.

C. The likelihood is something that exists now, not in the future.故will错误。另外will 和 be to有些重复
D. 我们要测量的是likelihood，正确。且这个测算是一直存在的客观事实，而不是一次性的，所以for更好
E. likelihood和may重复，且文中重点是为了测算，而不是着重强调develop

可不可以这么理解？
astronomers have developed a scale to rate 这里不定式to rate做宾语scale的宾补，动作rate的逻辑主语就是宾语scale，scale不能主动发出rate的动作，所以这里不合适；而for rating 则没有逻辑主语一说，规避了这一点

'to rate' would be more appropriate for an action that helps the astronomers toward their goal (e.g., To rate the likelihood that asteroids will xxxxxx, astronomers are measuring their current trajectories with exquisite precision.)

in any case, that's not the major problem with choice C. the major problem with choice C is that likelihood is a PRESENT thing, so 'will be' is inapposite.
i.e., the question the astronomers are trying to answer is "How likely is this asteroid to eventually strike Earth?" that's the probability at present.

C选项的will be to 很怪异

选了E，但是E中的likelihood和may应该是redundancy

rate the likelihood of ....为啥啥可能性打分。rate接宾语从句的话，打分的是可能性，不是宾语从句。另外to do ,for doing，这个感觉不好区分，这道题 to do 也能说得通啊，为了打分而研发了一个度量，这个不好判断，还是不要把这个原则作为首位的preference了

rate身后的宾语how likely a particular asteroid or comet will be to collide with Earth是宾语从句，其核心词是从句中的谓语动词collide（撞击）。但在逻辑上，真正被“打分”的对象应该是小行星撞击地球的“可能性”而不是“撞击”这个事件本身（打出来的分，肯定是某一个或几个小行星撞击的百分比嘛）。因此，rate宾语的核心词应是likelihood。

Manhatten 的 staff如此说：
for those of you who are not native speakers of english - the best approach to problems such as this one is:
* note the differences in usage between the formal and informal
- e.g., "rate how likely" vs. "rate the likelihood that..."
* remember what these differences look like, so that you can make similar distinctions in the future.

【注意：看到to do和for doing不要马上认为to do表目的，就把for doing给排了，先找其他绝对错误，有很多题for doing才是对的】
C： will be to还原后是will be likely to，不优；原句强调的是rate小行星撞击地球的“可能性”，rate宾语的核心词应是likelihood，在原句没有逻辑错误的情况下，基于原句的表意
E：逻辑上，应是“特定行星撞击的可能性”，而不是“特定行星的可能性”。因此，likelihood的修饰部分应用同位语从句。(GMAT loves同位语从句！)

E: likelihood .... may be语意重复？

C: 没有 future likely。 sb is likely to xx。而不是 sb will be likely to do; likelihood that sb will 。。。 makes sense
而且will be to collide 也很奇怪., you could have just said "will collide“ IMO
E:在逻辑上，应是“特定行星撞击的可能性”，而不是“特定行星的可能性”。因此，likelihood的修饰部分应用同位语从句。(GMAT loves同位语从句！)