Amir feels unbearable shame for mistreating Hassan. Amir feels as though Assef has been diminished by his own darkness. They were both comparable characters who just chose opposite paths, which is reminiscent of the hero/villain narrative in the movies they loved to watch. To demonstrate how Amir is developing and becoming more self-aware, Hosseini shares this dream.
A is true. B is false because you can use more than one, depending on the structure of the sentence. C is false because of grammar rules, and D is false because you can indeed use both, depending on the structure of the sentence.
