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

我选的A。不太明白的句子结构. 觉得 a theory用在这很奇怪。 a theory still debated 是和什么关联的呢。呵呵，不好意思，问得比较莫名其妙。还请牛人把这个题解释一下。