During the early years of European settlement on a continent that was viewed as “wilderness” by the newcomers, Native Americans, intimately knowing the ecology of the land, were a help in the rescuing of many Pilgrims and pioneers from hardship, or even death.

(A) Native Americans, intimately knowing the ecology of the land, were a help in the rescuing of

(B) Native Americans knew the ecology and the land intimately and this enabled them to help in the rescue of

(C) Native Americans, with their intimate knowledge of the ecology of the land, helped to rescue

(D) having intimate knowledge of the ecology of the land, Native Americans helped the rescue of（C）

(E) knowing intimately the ecology of the land, Native Americans helped to rescue

这也是OG179T，解释说B creates an awkward, redundant, fused sentence in which the first clause has to be repeated in the vague this of the second clause; furthermore, the comma required before and in larger compound sentences is omitted.

看到过很多说and前要加逗号的句子，请问大家到底什么时候要加逗号啊？

谢谢~~

thanks !!!