13. The voyage diaries were handed down by his great grandfather, so their brothers have kept them as ____ .

　　　 A. the apple in their eyes　　　　　　　　　　　　　　　　　　　　　 B. the apple of their mouths

　　　 C. the apple of their eyes　　　　　　　　　　　　　　　　　　　　　 D. an apple of their eyes