Read the following expressions chosen from academic introductions, and identify what part of an introduction each expression refers to. （1分）2.The increasing pollution of ground water resources by nitrites due to the anthropogenic activities from agriculture is receiving worldwide attention.
选项：

A:Background of the study;
B:Existing studies on the problem;
C:Problems with existing studies;
D:Research objectives;
E:Significance of the study of that problem;
F:Research contents; G:Description of the skeleton of the research