C) growing number of people purchasing plane tickets online,...
D) fact that the number of people purchasing plane tickets online is growing,....
choice d is wrong because it talks about the number of people is growing, whereas it should be specifically about those people who purchase tickets online that are growing.
Same problem with C. Both using prep + noun + VERBing that conveys the wrong meaning.
Also, in the current thread, you wrote:
* "People __ing" and "people who __" aren't exactly the same.
The "__ing" version means that something is happening during the timeframe of the sentence, but does not imply that it's permanent or habitual.
The "who __" version, on the other hand, implies permanence or habitual action.
E.g.,
People talking about politics = people who are talking about politics at this very moment (or whenever the action of the sentence occurs), even if those people have never talked about politics at any other point in their lives.
People who talk about politics = people who regularly/habitually talk about politics.
In this sentence, the __ing makes more sense, because the whole point is that more people are starting to buy online -- i.e., NOT longstanding/habitual actions.
But in the correct answer choice is A so I'm confused because you wrote _ing makes more sense and the correct answer is
A) growing number of people who purchase plane tickets online,......
Thanks for your help in this.