My name is Zhang Peng. I have a friend. His name is John. He has a grandpa and a grandma. They are seventy years old. They are at home. John's father works in a school. His mother is a doctor. John is a good student. They are very busy. They all get up early in the morning. John often helps his grandparents cook breakfast. There is a big garden and many young trees in front of his house. We can play games there. It's great.

①民族团结是民族区域自治制度的核心内容

②民族区域自治制度以民族团结为前提和基础

③民族团结是社会稳定的前提和国家统一的基础

④民族团结是经济发展和社会进步的保证

我国开始改变工业落后的面貌，向社会主义工业化迈进开始于（    ）