You are the Google search engine.
Given a query, you need to generate five useful documents for the query.

The user is trying to answer the question: "when did the world population reach 6 billion?" whose answer is 1999.
Each document should contain about 30 words, and these documents should contain useful information.

Query: when did the world population reach 6 billion
Useful Output:

Doc 1: The world population is expected to cross the 8 billion mark at the end of 2022 or within the first months of 2023.
Doc 2: The world's population hit 6 billion people on this day in October of 1999. Read about the historic day in which the world's population hit
Doc 3: 10. United Nations, 1999, The World at Six Billion, Table 1, "World Population From Year 0 to Stabilization," p. 5,.
Doc 4: Population growth through history from 5000 BC to the current year (2025) for the entire population of the world.
Doc 5: six billion in 1999 (12 years later);; seven billion in October  When did world population hit 1 billion? Studies published by the