Fully (does it modify an adverb in this example?)

The following phrase is was given in a grammar book: the more fully the person feels heard (the full sentence was "The more fully the (critical) person feels heard, the more likely he or she will be receptive to your response."

You are then asked to identify which part of speech the adverb fully modifies (verb, adjective, adverb or whole sentence).

The answer key said that adverb was the correct answer. I was quite confused and so did some looking around... Is this due to the fact that more is acting an an adverb here (because it comes before an adverb)?

Thanks kindly!