The 19th-century proponents of the school of thought known as mechanism held that life process are not the products of some mysterious life force, but are the same chemical and physical processes that operate in inorganic
systems, which is still a subject of debate between biologists today.
(A) systems, which is still a subject of debate between - which illogically refers to inorganic systems ; also verb "is" is incorrect for plural systems
(B) systems, which are still debated between - which illogically refers to inorganic systems
(C) systems still debated among - Illogical meaning - debated modifies systems
(D) systems, a theory still debated by - Correct
(E) systems, a theory still debated between - Idiom error - between X and Y
1. Isn't there a subject-verb agreement issue - life process is singular and thus should take verb 'is'?
2. Proponents held that life process are not X, but
are Y that ... --> shouldn't the parts X and Y be parallel? Do we need to repeat the verb in part that follows "BUT" ?
3. Comma+ FANBOYS is used to join two independent clauses- but here the 2 clauses aren't independent.
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Answer D