Joe Read studied in this school for four years. When he finished school, he was already eighteen years old. And then his father said to him, “You finished school, and you are a good student. Now you may go to town and get a good job. They need some clever people to work in the office. The people there can get a lot of money now. If you stay at home, you can't get money from our family.” A few weeks later, Joe went to the office and asked for a job there. A man took him into a small room and gave him some questions on a piece of paper. Joe answered the questions quickly, and he gave the paper to the man. The man looked at the paper for a few minutes and then asked, “You were born on Sep. 23. But which year were you born in?” Joe answered, “Oh, every year.”

一个物体在地球表面所受的重力为G，则在距地面高度为地球半径的2倍时，所受的引力为（   ）