The population (人口) problem may be the (41) one of the world today. The world’s (42) is growing fast. Two thousand years ago, there were only 250 million people (43) the earth. Four hundred years ago, the number was (44) 500 million. But at the beginning of (45) century, the world’s population was about 1,700 million. In 1970, it was 3,600 million, and in 1990, 5,000 million. A (46) report says the world population will (47) 6,000 million by the end of the 20th century. This is just ten (48) after it (49) five thousand million. People say by the year 2010, (50) may be seven thousand million.