I think it might be "They portray time as ruthless, taking children into adulthood when they least expect it."
The answer is: A: They show that the speaker didn't appreciate his childhood until time took it away.
At long last, we see regret creeping through these lines. The speaker says he didn't care to heed such beauty and innocence back then, and didn't realize that those days would be "so few." - Shmoop.com