D). This grading difference creates an unfair comparison between students.
Explanation:
The last statement i.e. 'this grading difference creates an unfair comparison between students' can be added to the end of third paragraph to back the ideas presented in the paragraph. This statement is functioning as the conclusion of the ideas offered in the text as it reiterates the key ideas of the argument precisely and wraps up the paragraph effectively. Thus, it not only supports the idea that 'the artificial raising of grades is proposing an unfair advantage to the students' but also reinforces it into the readers mind that 'difference in grading leads to a biased and discriminatory comparison among students.' Thus, option D is the correct answer.
