The sun had gone behind a cloud. I was very tired and wanted nothing in the world so much as to be at home. At last I got to the gate of Hide Park. But this was worse than ever; there were buses there--high and terribly red cars, taxis and still more buses in an endless line. Everywhere there were people hurrying past or waiting to get into the buses, while I stood lost in the middle of them. I was ready to cry. In despair, I crossed the street on to an "island", where I found a policeman. I took my last bit of courage in both hands and said, "Please, sir, where is Addison Road?" He began to explain, but when he saw that I couldn't understand he became helpless, too. "Are you French at school?" A few minutes later, he smiled and raised his hand. How wonderful! The traffic stopped. Even the red buses stood still and waited until I had crossed the road. In this passage "island" means  _  .
a piece of land surrounded by water a raised place in a busy street where people may be safe from traffic a safe place that can only be used by policemen a safe place that nobody can use without permission