prep语法中的一个困惑

13.   (25151-!-item-!-188;#058&002234)

One of the earliest known birds with a beak and contour feathers, Confuciusornis sanctus, with large clawlike "thumbs" on its wings, which probably helped them to climb up to a launching position for flight.

(A) with large clawlike "thumbs" on its wings, which probably helped them to

(B) with large clawlike "thumbs" on their wings, which probably helped it to

(C) had large clawlike "thumbs" on its wings, which probably helped them

(D) had large clawlike "thumbs" on its wings, probably to help it

(E) had large clawlike "thumbs" on their wings, probably to help it

这题中probably to help it应该是作状语吧，那为什莫要加个逗号呢？不加可不可以呢，加与不加有区别吗？好像prep中很多作状语的都在前面加个逗号，对prep的这个独特的现象很困惑，请NN解惑，多谢