It is a compound noun. Why? Because 'Back''Pack' is one word, but if you see how i structured it, it can be two. Back, as in your body part, and Pack as in a bag or container, you name it. And just a tip, compound means two words squished into one, ex. CupCake, FireWorks, BaseBall, BasketBall. But when you are typing it, of course, you wouldn't capitalize the first letter of the two words :)
In order to answer this we need to see the 'following' words
Answer:
D.passageway..
Explanation:
A corridor is a form of hallway or gallery
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