Often it is obvious what word is being modified by a prepositional phrase, but sometimes it isn't. When it's not immediately clear, I often ask myself questions like: Does this phrase answer "where _______" or "when _______" indicating adverb phrases or "which _________?" etc., indicating adjective phrases. Or sometimes I eliminate words and that often reveals the answer.
However, in the following sentence, I was not able to tell what words were being modified by asking questions or eliminating words.
"Are you happy about the grade you received in biology?" (The prepositional phrases are in italics.)
According to the answer book, about the grade modifies "happy." That makes sense, although I wouldn't be able to explain why.
In biology apparently modifies "received", not "grade." That isn't very apparent to me. One could make the argument that in biology is answering the question "which grade?"
How can we tell in this case? Both options seem reasonable. Are there questions we can ask ourselves, in this case, that would help us discover what word is being modified?
Or perhaps the answer book is in error ...