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