Mr and Mrs Green married thirty years ago and they have lived in the same house since then. Mr Green goes to work at eight every morning from Monday to Friday and Mrs Green does the housework everyday. Both of them enjoy their work very much. There are quite a lot of houses in their street, and most of their neighbors are kind and friendly. One day, the old lady in the house opposite them died and, after a few weeks, a young man and a young woman came to live in it. Mrs Green watched them for a few days from her window. One day when her husband came back from work, she said to him, "Bill, the man in that house opposite us always kisses his wife when he leaves in the morning and kisses her again when he comes home in the evening. Why don't you do that, too?" Mr Green thought for a while and answered, " Well, I don't know her very well yet." From then on, Mrs Green never mentioned it again. From the passage, we can see that Bill is    _    .
the young man Mr Green the young woman Mr Green's son