Sentence 1: Olivia had sent birthday party invitations to all of her friends. 
 Sentence 3: She was starting to feel as if her best friend didn't care 
 Sentence 4:  Meanwhile, her friend was sad for she had not received an invitation 
 Sentence 5:  When the lost mail finally arrived, the misunderstanding was resolved! 
 Given Sentence 2: She had heard back from everybody except for her very best friend!