"I hear it's someone's birthday today," remarked Ms. Perkins, the secretary, as Benjamin exited his mother's office. This was a disadvantage of having your mom as your school principal—the faculty knew everything about you.
"I'm eighteen today," Benjamin replied with an embarrassed grin.
"Well, you don't look any different than you did yesterday," Ms. Perkins joked as she signed his hall pass.
Later, Benjamin caught his reflection in the cafeteria window and thought about what Ms. Perkins had said. Today, he was officially an adult, but he didn't look any different. He still had the same dark, disheveled hair and the same inquisitive eyes that were so large they sometimes seemed disproportionate with the rest of his face. He was the same height, and he still had the same prominent chin.
He also didn't feel particularly different. Benjamin could vividly recall the exhilaration he'd experienced on the morning of his tenth birthday. He'd bounded out of bed and basked in the knowledge that he was now one of the big kids. Benjamin felt as if he could do anything in that moment, but he didn't feel that immeasurable sense of joy today—and he couldn't help but wonder why.

Why does the author use flashback in the passage?
A) To create a humorous tone
B) To introduce the main characters
C) To establish the setting of the passage
D) To highlight how different birthdays feel