(A) it takes three to four years for the mountain yellow-legged frog of the Sierra Nevada to reach adulthood, and so they are

(B) it takes the mountain yellow-legged frog of the Sierra Nevada three to four years until it reaches adulthood, and therefore it is

(C) in the Sierra Nevada, mountain yellow-legged tree frogs take three to four years to reach adulthood, thus being

(D) mountain yellow-legged frogs of the Sierra Nevada take three to four years until they reach adulthood, thus

(E) mountain yellow-legged frogs of the Sierra Nevada take three to four years to reach adulthood, and so they are

我还是很怀疑这个 and so 的用法，之前大家说这里so是副词，但是如果so是副词就没有“所以”这层意思了，也就是没有表达出前一句和后一句的因果关系啊？
请NN指正！我非常之疑惑！