Problem: The costume Megan wore was a mystery to Rebecca , so

My choice: Megan would not reveal her true identity.

Problem: The essay of Felicia was more popular than Natalie's since

My choice: Felicia devoted a lot of time to the paper.

Problem: Elena has been arrested twelve times while Jessica has never been arrested because

My choice: Jessica avoids having run-ins with the police.

Problem: Elena passed the exam while Laura failed because

My choice:
Elena studied hard for the exam leading up to it.