Mr Brown saw a sweater in a shop window. It was very cheap, so he bought it. When he got home, he put it on. It was all right and he was pleased with it.
   In the afternoon, he went to work in his garden. It soon began to rain and Mr Brown had to run into the house. He ran quickly, but he still got wet. Then his sweater started to shrink(收缩)! It got smaller and smaller. Mr Brown tried to take it off, but he couldn't.
In the end, with the help of his wife, he got out of the sweater. Mrs Brown laughed, “You see, you bought a cheap thing, but…”

实际划分东西半球的界线是（    ）