Problem: Ian was in the library helping Samuel solve the chemical equation since

My choice: Ian was a perfect student.

Problem: With lots of persistence, Katrina finally caught Megan, so

My choice: Katrina got paid for the bounty.

Problem: Erin was always trying to push Sarah to follow correct grammar, because

My choice: Erin was controlling and anal retentive.

Problem: Joel uses coke to clean his silverware, but Nelson didn't know that was possible.

My choice:
Nelson is unaware.