prep-1-95:十分弱的问题"...,and so"的用法对么？

95.   (29661-!-item-!-188;#058&003962)

Unlike frogs that metamorphose from tadpoles into adults within a one-year period, it takes three to four years for the mountain yellow-legged frog of the Sierra Nevada to reach adulthood, and so they are restricted to deeper bodies of water that do not dry up in summer or freeze solid in winter.

(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

选E，我看 and so不爽，于是选了D。。。

and so两个都是conj,可以这么用么？

so

shy//我错了，，，飘过