代词的问题。怎么也整不明白了

To meet the rapidly rising market demand for fish and seafood, suppliers are growing fish twice as fast as their natural growth rate, cutting their feed allotment by nearly half and raising them on special diets

A) their natural growth rate, cutting their feed allotment
B) their natural growth rate, their feed allotment cut
C) growing them naturally, cutting their feed allotment
D) they grow naturally, cutting their feed allotment
E) they grow naturally, with their feed allotment cut

这道题挺老了吧。但总是搞不懂。为什么D就是no human intervention，C中就没有这个意思呢？

请大牛解释~

到底是谁代指谁。they their ,头晕了。

用逻辑意思来解答

C  suppliers are growing fish twice as fast as （suppliers are )growing them naturally

这如何能行得通呢

比较的对象是 人工养殖的鱼和自己自然生长的鱼生张速度的比较