Question

reese needs a better transition between the second paragraph (sentences 4 - 8) and the third paragraph (sentences 9 - 14). which sentence should replace sentence 9 to create a more effective transition?
the lymphatic system does not include the brain, so everyone always wondered about that.
scientists researched the brain for years before finally making a breakthrough.
no one knew how the brain was able to perform some functions.
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.

Explanation:

The second paragraph likely discusses the lymphatic system (as sentence 9's replacement options mention it). The third paragraph (sentences 9 - 14) probably talks about the brain's waste - removal. The option "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" connects the lymphatic system (from the second paragraph context) with the brain's waste - removal (third paragraph topic). It provides a reason (lymphatic system exclusion) for the scientific mystery (brain waste - removal) which is a good transition. The first option just states a fact about the lymphatic system and a wonder without connecting to the brain's function. The second option focuses on brain research in general without the lymphatic - brain waste connection. The third option is too vague about brain functions without the lymphatic system link.

Answer:

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.