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Researchers now regard interferon as not a single substance, but it is rather a biological family of complex molecules that play an important, though not entirely defined, role in the immune system.
A. as not a single substance, but it is rather a biological family of complex molecules that play
B. as not a single substance but as a biological family of complex molecules playing
C. not as a single substance but as a biological family of complex molecules that play
D. not to be a single substance but rather a biological family of complex molecules playing
E. not as a single substance but instead as being a biological family of complex molecules that play
C
A. as not a single substance, but it is rather a biological family of complex molecules that play
B. as not a single substance but as a biological family of complex molecules playing
C. not as a single substance but as a biological family of complex molecules that play
D. not to be a single substance but rather a biological family of complex molecules playing
E. not as a single substance but instead as being a biological family of complex molecules that play
C
