A、At the age of 31. B、At the age of 36. C、At the age of 20. D、In 1986. B题目询问比尔·盖茨什么时候成为世界首富。关键是听到“31岁时成了千万富翁，五年后，位居世界首富”，可计算出选项B

Gates was born and brought up in Seattle. [29]At the age of 14, he founded a computer programming company with three friends and they had earned $20,000 by selling their traffic-counting system to local governments. In 1975,  he dropped out of his law course at Harvard to found the Microsoft Software Company in Washington. Gates" domination of the emerging computer industry began in 1980-1981, when he devised an operating system and licensed it to IBM. [30]MS-DOS became the standard operating system for nearly all IBM personal computers. During the 1980s, Microsoft also developed more specialized software. When the company went public in 1986,[31]Gates became a multimillionaire at the age of 31. Five years later, he was ranked as the world’s richest man. In the 1990s, Gates made a fresh fortune from sales of Windows, a system that enables a computer to be operated with on-screen symbols rather than complex keyboard commands. A revised version was launched amid huge publicity in 1995.