BEIJING — China has executed a serial killer convicted of murdering 11 people, including three police officers, over the course of nearly a decade. Cheng Ruilong, 37, was sentenced to death earlier this year by the Foshan Municipal Intermediate People’s Court for murder, robbery and rape in China’s southern province of Guangdong, the official Xinhua News Agency reported Wednesday. During Cheng’s appeal to the Supreme Court, the court reduced the number of victims from 13 to 11 because the bodies of a mother and daughter allegedly killed by Cheng were not found, the report said. Cheng smiled when his sentencing was read out at the Supreme Court, according to the state-run Guangzhou Daily. He was executed by lethal injection Tuesday, state media reported. Calls to the Foshan Municipal Intermediate People’s Court rang unanswered Wednesday.