Whose picture does ponyboy see while going through sodapop's old yearbook? why doesn't he recognized the person immediately

Pony sees a picture of a boy but realizes it's Bob's (Robert Sheldon) picture in Soda's old yearbook and starts to wonder about Bob as a person. He thinks that he is equally guilty for killing Bob because he was at the scene. He wonders about the Bob Cherry Valance knew. He wonders whether Bob has a brother. And wonders how it was that Bob could be the best friend a person could ever have according to Randy. He wonders How Bob's parents were handling his death, but the general idea is seeing Bob as a real person.  

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