请教大全24

                             Just as plant species native to regions with browsing mammals evolved many natural anti-browser defenses (such as sharp spines and toxic chemicals), so humans in malarial regions have evolved dozens of chemical defenses against malaria.

(A) so humans in malarial regions have evolved dozens of chemical defenses against malaria

(B) humans in malarial regions have been evolving dozens of chemical defenses against malaria

(C) there has been, in malarial regions, an evolution of dozens of human chemical defenses against malaria

(D) dozens of chemical defenses against malaria have been evolved by humans in malarial regions

(E) similarly, in malarial regions, humans have evolved dozens of chemical defenses against malaria

请问为什么a对啊？我也觉得其余答案不对，但是a中为什么可以用so引导，一个句子中两个分句都可以用连词引导的吗？