Because Halley’s comet changes orbit slightly during the seventy-six

C is correct.

The subject of the sentence is "Halley's comet", but the sentence describes the path of the comet. A pronoun is required to refer to "comet's path/pass"
"its" correctly refers to Comet's passes or passing (the comet's action)

The options are narrowed down to C and D. D is better. "between" correctly describes the comet's action between 2 stages.

I too chose E, can someone explain more why C is good? some explanation is there above but kind of confusing really.

Here in subordinate clause, its refer to comet's not comet's orbit's. Comet's orbit doesn't pass in interval close to Earth, but the comet does.

how to define an interval - (interval between two passes of comet close to Earth[*consecutive is implicit]) <== This is what is implied in here by using "between its passes".

Also, 'it' in last clause again refers to Comet not Comet's orbit.

Step 1) Figure out what the sentence is trying to say. Basically there's a 76 year waiting period between each pass.

Step 2) Recognize that "between passing" doesn't make sense. I was initially looking for something in the form of "between passing [X] and verb-ING [Y]" but did not see any of that.

Step 3) Glance through answer choices to see what sounds right. Although (E) sounds right, it changes the original meaning of the sentence. With (E), it's saying that during the 76 years, the comet passes the Earth. It's like saying it takes 76 years for the comet to finish passing the Earth--this doesn't make sense.

Step 4) What we're trying to say is that the comet passes....and then there's a silent period of 76 years...and then the comet passes again. Specifically during this time period "between its passes" blah blah blah.

Step 5) So "between its passes" is most appropriate. Answer (C)

OA should be C.

In E, "as it passes" imples that the Comet takes 76 yrs to pass close to Earth. If thats true then people will book trips in advance

So, E is definitly wrong. We are discussing about an interval, which is calculated b/w 2 occurances. So, I think C is correct.

the sentence talks about the change in comets orbit leading to a future possible collision.

I earlier thought if the sentence said 'Because Halley’s comet changes orbit slightly as it passes close to earth,it may.............' here as is used to signify the time when the comet changes orbit. But then 'during the 76 yr interval' wouldn't fit in. 76 yr interval in what?

but may be the author is trying to say -- that the comet passes earth after the interval of every 76 years and during this period it changes course slightly , which may cause a collision........
So this seems to make sense. So IMO it is D (though 'of its passes' sounds bad) passes=when the comet passes earth the 2nd time after 76 yrs
Because Halley’s comet changes orbit slightly during the seventy-six-year interval of its passes close to Earth, it may veer onto a collision course with a planet sometime in the distant future.


Editing my post-- just saw the explanation by Stacy Koprince and a little caveat was added 'Please do not study stuff that you find on random web sites without sourcing or attribution as to the author of the material. If something feels weird or not GMAT-like, don't study it. '

accordinng to Oxford dictionary, "interval between" two events and "interval of" a period of time
eg: interval between a flash of lightning and the sound of thunder, interval of half an hour

I have initially chosen E, while the correct answer is C, but now I see the difference.

Because Halley’s comet changes orbit slightly during the seventy-six-year interval between passing close to Earth, it may veer onto a collision course with a planet sometime in the distant future.

(C) between its passes - implies that the comet changes its orbit between times it passes close to Earth, which makes more sense. I.e. scientist have measured comet's orbit twice in 1900 and then in 1976 at times it passed close to earth. They realized that the comet's orbit has changed between 1900 and 1976
Vs.
(E) as it passes - implies that the comet changes it's orbit in a very moment it passes close to Earth, which makes no sense. Comet can't change its orbit instantly in the moment it passes close to Earth.

What the original sentence is : Comet can collide with a planet because it changes orbit between its passes close to Earth.
Intended meaning of original sentence : Comet can collide with a planet.
Why ?
=> Because comet changes its orbit in which it moves.

When comet does so ?
=> When comet is orbiting close to earth, at that time between its passes comet does so.


Because Halley’s comet changes orbit slightly during the seventy-six-year interval between passing close to Earth, it may veer onto a collision course with a planet sometime in the distant future.

(A) between passing => we need something to refer back to comet in order to tell who passes
(B) of passing => passing of what ?
(C) between its passes => CORRECT. (discussed above)
(D) of its passes => comet changes orbit during the interval of its passes
Passes is a superficial thing. Interval can not belong to passes. Hence it is incorrect to say 'interval of passes'. Between its passes conveys the intended meaning.
(E) as it passes => This is tricky. Here, during interval as it passes => both underlined parts convey simultaneous ongoing actions because 'as' is used to mean 'while' it passes. This changes the intended meaning or original sentence.

Hence C is the correct answer.

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