Which word in the sentence is the predicate adjective? the atlantic ocean must have seemed endlessly vast to early sailors. a. vast b. atlantic c. endlessly d. early

A. Vast

The predicate adjectives are adjectives that change the subject. In this case, the subject is the Atlantic Ocean and so the adjective that modifies it is the predicate adjective. In this sentence, "vast" is the one that changes the Atlantic Ocean.