Answer:
Hey mate, here's ur answer
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The ryt answer is option A. messy
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A subject complement is a word or phrase that follows a linking verb (a verb used to link a subject to a new identity or description) and identifies or describes the subject.
Room being the subject, messy is describing the room and terribly is linking the subject (room) and messy.
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Hope this helps
#stayhomestaysafemate
:D
