Question

Please answer! 1. How is the infinitive used in the sentence? Was that gigantic sandcastle difficult to build?
a. noun
b. adverb
c. adjective

Answer 1

The correct answer is: adverb.

It's also referred to as adverb infinitive. It modifies "difficult", which is a predicate adjective (it gives more information about the subject of the sentence and there is a linking verb).