Halley's Comet

[ranjeet1975]

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
b. of passing
c. between its passes
d. of its passes
e. as it passes

I think the answer should be E, but the OA is C.
The use of 'its' and 'passes' right here:
'its' refers to what?

[Ben Ku]

Please cite the source (author) of this problem. We cannot reply unless a source is cited (and, if no source is cited, we will have to delete the post!). Thanks.