Among the given options, the one that has the most effective transition to replace sentence 9 in the text is the following:
A. Until recently, scientists could not determine how the brain is able to rid itself of waste because the lymphatic system does not include the brain.
- The general topic of Reese's essay is the brain. However, he spends the whole second paragraph talking about the lymphatic system.
- In the third paragraph, he goes back to talking about the brain, but no transition is used to connect the two ideas, that is, the lymphatic system and the brain.
- Therefore, we need a transition that shows how both ideas are related.
- We can eliminate options B and C, since they do not do that.
- We are left with A and D. They both show that Reese wants to explain that the lymphatic system does not include the brain, which left scientists wondering.
- However, option D is too informal for this essay.
- In conclusion, the best option is letter A.
Learn more about the topic here:
https://brainly.com/question/22106814