若本金为P，年利率为r，每年的计息次数为m次，则n年末该投资的期值计算公式为( )。

      It's Monday today. The young couple (夫妻) are at work and their children are at school but their
parents are retired at home.
      "It's your birthday today, dear." Mrs. Morison says."Let's go shopping and make a birthday dinner
at home." The old man agrees with her. So they go out. But it's very cold outside. Most shops are closed
and they can' t buy any vegetables or meat. When they walk by a small restaurant, Mr. Morison says,
"Well, then, Let's go in and eat something there."
      The old woman thinks he's right and they come in. There's nobody but a girl.
      "Can I help you?" asks the girl.
      "A fish, four eggs and some chicken and bread," says Mrs. Morison. 
       Half an hour later, the girl brings all their dishes to them and Mrs. Morison says, "Your birthday,
dear…" She asks her husband to eat first and she watches him.
       The girl comes over and feels strange (奇怪的) and asks, "Aren't you hungry, madam?" "Yes, I am
hungry," says the old woman. "But I have to wait. He's using my false teeth (假牙) now."