Answer:
C
Explanation:
I have never had this question yet C makes the most sense. ( I dont know 100)
I will message you the answer
The t-shirt that they give only to people who finish is a big reason that so many people take up the challenging race.
This paraphrase presents the same information as in the sentence, but since it should be a bit more of a summary, it takes out the specific detail of how many people actually run the race, which can just be paraphrased into 'so many'.