“…The task is to answer a question with external knowledge bases without any given context paragraphs [43]. There are mainly two streams of approaches, namely knowledge graph-based question answering [10,31,37,51,58,64] and knowledge retrieval-based question answering [5,7,18,28,43,44,59,61]. For retrieval-based methods, both elastic-search such as BM25 [45] and semantic search such as Dense Passage Retrieval (DPR) [23] are utilized to retrieve most relevant knowledge snippets from knowledge bases.…”