In The Hobbit by J. R. R. Tolkien, Bilbo is initially described as a weaker character, prone to fits of fainting at the slightest hint of danger. But as the story progresses, Bilbo demonstrates profound inner strength and acts with great courage in the face of challenges. Based on this synopsis, what theme is likely used to develop Bilbo's character in this story?
death and rebirth
salvation
love and duty
heroism