What are the possible contents of an introduction?

A.Reviewing the existing literature relevant to the present study

B.Announcing the purpose/focus of the study

C.Identifying a gap in the existing literature

D.Explaining the significance or necessity of the research

正确答案：Reviewing the existing literature relevant to the present study；Announcing the purpose/focus of the study；Identifying a gap in the existing literature；Explaining the significance or necessity of the research