Although ____________, skin is often viewed as simply a barrier between the body and the outside world.
a. a part of the immune system
b.there is a part of the immune system
c.it is the part of a immune system
d.the immune system is a part of it
여기서 although + s + be +n ,skin
이렇게 들어갈수 있잖아요 s = n= skin 동격
이라는 전제는 왜 생기는건가요 그리고 although 다음에 들어가는게 주어동사, 형용사., 명사., 전치사구, be +s +v 가있는데 이것들이 들어가는 기준이 뭔가요
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