听力原文： Chinese government is poised to conduct its first national survey of pollution sources in February to help control environmental deterioration in the country. The study will identify and collect data on sources of industrial, agricultural and residential pollution for two months. Last year, China's environment was facing a grave situation, with several major rivers and lakes clogged by industrial waste. China's environmental cleanup is compromised by more than two decades of rapid economic growth, and a lack of technology especially. Every province, autonomous region and municipality has set up a census office and will report to a main center staffed by officials from government departments. Data will be reviewed multiple times before being put into a database and will be analyzed in the second half of 2008. Findings will be examined and approved by mid-2009. What is the purpose of the national survey?