The word "either" is a correlative. The answer is D. This is because a correlative conjunction connects two nouns and ideas that relates to each other. Either uses "or" so it connects two choices in one situation.
C is the complex sentence because it contains the word while and an independent and dependent clause! :)
I strongly believe that it is B. I know it isn't D because it doesn't compare or contrast asthma to something else.
