6. 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

(B) systems, which are still debated between

(C) systems still debated among

(D) systems, a theory still debated by（D）

(E) systems, a theory still debated between

不明白C为什么不对呢？ A B C 错是不是因为which引导的定语从句不能指代前面的句子？