单项选择题

For all problems described above, the goal is to find a trajectory (a flight path) such that the mission success is as large as possible.
A、A. Explaining the significance or necessity of the research
B、B. Reviewing the existing literature relevant to the present study
C、C. Identifying a gap in the existing literature
D、D. Announcing the purpose/focus of the study
E、E. Outlining the paper structure