题目内容：
There are several groups which have analyzed the performance and yield of PV systems with a systematic approach that focuses on the specific yield and performance ratio of these systems.
A.Explaining the significance or necessity of the research
B.Reviewing the existing literature relevant to the present study
C.Identifying a gap in the existing literature
D.Announcing the purpose/focus of the study
E.E.Outlining the paper structure
参考答案：