SHANGHAI, Oct 29 (Reuters) - China’s central bank on Friday injected 200 billion yuan ($31.29 billion) through seven-day reverse repurchase agreements into the banking system, bringing the weekly net cash injection to the highest in 21 months. The People’s Bank of China (PBOC) said its website said the move was to “maintain stability in month-end liquidity conditions.” With 100 billion yuan worth of reverse repos maturing on Friday, the central bank has injected 100 billion yuan on a net basis on the day. For the week, the PBOC has net injected 680 billion yuan through reverse repos, biggest weekly cash offering since January 2020. ($1 = 6.3917 Chinese yuan) Reporting by Winni Zhou and Andrew Galbraith; Editing by Christian Schmollinger