Phelps has won the most Olympic gold medals. He is believed to be the greatest swimmer in the world, (complex sentence)
【正确答案】：Phelps,who has won the most Olympic gold medals,is believed to be the greatest swimmer in the world.
【题目解析】：本句的要求是将两个单句变成一个复合句（complex sentence）。我们可以将其变成一个定语从句who has won the most Olympic gold medals。