Select the correct word(s) to make the sentence grammatically correct. Two students ___ their professor about the possibility of obtaining a different grade in class.

The correct answer is D: "had asked." This is because the sentence indicates a past action that occurred before another past action. "Had asked" is in the past perfect tense, indicating that the students asked their professor before another event in the past. The other choices (A, B, C) are in present or past simple tenses, which do not convey the sequence of events correctly. Choice A "asking" is present tense, which does not fit with the past context. Choice B "has asked" is present perfect tense, indicating an action that started in the past and has relevance to the present, which is not the intended meaning. Choice C "was asking" is past continuous tense, suggesting ongoing action in the past, which is not suitable for the context of the sentence.