Mary is a little girl. She is only four. She is not at school. She doesn't know how to read or write. But her sister Joan is a schoolgirl. She is ten years old. She knows how to read and write. One day, Joan sees her little sister in the room. She's at the table. There's a pencil in her hand. She's writing. “What are you writing, Mary?” She asks. “I'm writing a letter to my friend Rose. ” “But you don't know how to write. ”says her sister. “Well,” says Mary, "it doesn't matter. Rose doesn't know how to read. "

①肝脏  ②脾   ③肾脏  ④肺  ⑤胰腺．

下列有关光现象的说法中，正确的是（　　）