I originally selected choice B for item 21, thinking it aligned with the general benefits of thinking back (a review) as discussed in the passage. I remembered that there was a sentence about improvement, and I thought choice B fit this premise. However, upon review, I found choice C was a better fit. Choice C talks about "thoughtfully examine the possible causes," and the passage expresses the same idea by explaining that we need to "review contributing factors in a thoughtful way."