from langchain.chains import RetrievalQA
from langchain.vectorstores import Chroma

qa_chain = RetrievalQA.from_chain_type(
    llm=llm,
    retriever=vector_store.as_retriever(),
    return_source_documents=True
)

result = qa_chain({"query": "什么是 RAG？"})
print(result["result"])