关于总星系的说法，正确的是（　　）

①可见宇宙  ②已知宇宙  ③半径约140亿光年  ④半径约140亿年  ⑤半径约140亿千米  ⑥主要天体系统为银河系与太阳系．

根据短文内容，完成下列小题。

    The story is about a young student. She worked very hard at her lessons. She worked so hard that she became ill. She couldn't go to sleep at night. When she got up the next morning, she felt very tired.

    So she went to see a doctor. Hearing the student's words, the doctor said, "You are too stressed out. You must eat more food like vegetables and tofu. Besides you can try to count(数) numbers when you are lying in bed. By the time you reach one thousand, you will be asleep. I'm sure of it."

    The next day, the student came to see the doctor again. "Well," said the doctor, "how are you feeling today?"

    The student still looked tired. "Yes," she said, "I tried to count one, two, three, four... up to one thousand. But when I reached five hundred and sixty, I feel sleepy. I had to get up and drink some coffee so that I could go on counting up to one thousand. But then I still couldn't be asleep."

发达国家与发展中国家的划分依据是（　　）