Hi All,

Rahul's explanation is correct, but I'll add some information that should help to clarify it. Assuming you're choosing between D and E....

From an idiom standpoint, you can't "rate the likelihood" that something MAY happen, you're "rating the likelihood" that it WILL happen.

Also, answer E uses the phase "that may....", which is redundant (it's clear that the SC is discussing an asteroid or comet).

Both options point to answer D

GMAT assassins aren't born, they're made,
Rich

sinsofgmat wrote:Could you please help us differentiate between rates and rating. If "rates the likelihood" is incorrect then why "rating the likelihood" is acceptable?

The split between "to rate," "for rating," and "that rates" is actually not significant. These are all idiomatically correct!

The real issue here is one of MEANING. What are we rating the likelihood of? Of asteroids COLLIDING with Earth. We could say any of the following:
developed a scale for rating the likelihood that a particular asteroid or comet will collide
developed a scale that rates the likelihood that a particular asteroid or comet will collide
developed a scale to rate the likelihood that a particular asteroid or comet will collide

It's really a question of LIKELIHOOD THAT v. LIKELIHOOD OF. "Likelihood that" will set up a clause, so we're talking about the likelihood of the act of colliding. "Likelihood of" sets up a noun, so we're talking about the likelihood of the asteroid.

In E, "a scale that rates the likelihood of a particular asteroid or comet that may," it's the likelihood of an asteroid - this doesn't make sense. We want the likelihood of the collision.

In C, "rate how likely [it] will be to collide" is both wordy and not idiomatically correct. The larger issue, though, is that we're again talking about the likelihood of the asteroid, and not of the collision.