Chinese star Yao Ming has retired. Yao made it official Wednesday, telling a news conference that a series of foot and leg injuries forced him to end his playing career at the age of 30. "I will formally end my career,'' said Yao, who became a household name in China before starting his NBA career with the Houston Rockets in 2002. Yao played eight seasons in the NBA. "Today is an important day for me and holds a special meaning for both my basketball career and my future,'' Yao said. "I had to leave the court since I suffered a stress fracture  in my left foot for the third time at the end of last year. My past six months were a painful wait. I have been thinking about my future over and over. Today I am announcing a personal decision, ending my career as a basketball player and officially retire. But one door is closing and another one is opening.'' Yao said he would return to work with his former Chinese team, the Shanghai Sharks, with the possibility of becoming general manager. He already owns the club and wants to contribute more. "My playing career started with the club. I hope I can do something for it,'' Yao said. He later appeared with his family on the stage to the applause and cheers of the crowd. He thanked a lot of people -- his family, former coaches, even players like Shaquille O'Neal -- "for making me a better player.'' Rockets general manager Daryl Morey was among those attending the farewell  conference Wednesday. Morey made the long trip from Houston. Morey said 20 hours on planes was tiring but "I would be sorry if I wasn't here." "It's a big moment,'' Morey added. "Yao had a sense of humor, a great attitude and sense of responsibility. I hope we can continue his culture in the NBA.'' Yao Ming has retired from basketball because of   _  .
his age his bad conducts on the court his foot and leg injuries his poor health
Question: Chinese star Yao Ming has retired. Yao made it official Wednesday, telling a news conference that a series of foot and leg injuries forced him to end his playing career at the age of 30. "I will formally end my career,'' said Yao, who became a household name in China before starting his NBA career with the Houston Rockets in 2002. Yao played eight seasons in the NBA. "Today is an important day for me and holds a special meaning for both my basketball career and my future,'' Yao said. "I had to leave the court since I suffered a stress fracture  in my left foot for the third time at the end of last year. My past six months were a painful wait. I have been thinking about my future over and over. Today I am announcing a personal decision, ending my career as a basketball player and officially retire. But one door is closing and another one is opening.'' Yao said he would return to work with his former Chinese team, the Shanghai Sharks, with the possibility of becoming general manager. He already owns the club and wants to contribute more. "My playing career started with the club. I hope I can do something for it,'' Yao said. He later appeared with his family on the stage to the applause and cheers of the crowd. He thanked a lot of people -- his family, former coaches, even players like Shaquille O'Neal -- "for making me a better player.'' Rockets general manager Daryl Morey was among those attending the farewell  conference Wednesday. Morey made the long trip from Houston. Morey said 20 hours on planes was tiring but "I would be sorry if I wasn't here." "It's a big moment,'' Morey added. "Yao had a sense of humor, a great attitude and sense of responsibility. I hope we can continue his culture in the NBA.'' Yao Ming has retired from basketball because of   _  .
A. his age
B. his bad conducts on the court
C. his foot and leg injuries
D. his poor health
Answer: