The population problem may be the greatest one of the world today. The world’s population is growing faster and faster. Two thousand years ago, there were only 250 million people on the earth. Four hundred years ago, the number was over 500 million. But at the beginning of the 20^th century, the world’s population was about 1,700 million. In 1970, the number was3,600 million. In 1990, the number was five billion. A UN report said that the world population had passed six billion by the end of the 20^th century. This was just ten years after it reached five billion. By October 31, 2011, with a new baby’s birth, the world’s population has reached 7 billion. But another billion is coming by 2025, and the total will reach 10 billion before the end of the century.
根据短文内容，完成下列各题。每空填一词。

Time	World’s population
Two thousand years ago	There were only _______ million people on the earth.
Four hundred years ago	The number was _______500 million.
At the _______of the 20^th century	The world’s population was about 1,700 million.
By the end of the 20^th century	The world population had _______six billion
By the year of 2025	The world’s population will be ________ billion.