prep1-158 并列

158.      (33377-!-item-!-188;#058&006720)

The largest trade-book publisher in the United States has announced the creation of a new digital imprint division, under which it will publish about 20 purely digital works to be sold online as either electronic books or downloadable copies that can be printed upon purchase.

(A) works to be sold online as either electronic books or

(B) works to sell them online, either as electronic books or

(C) works and it will sell them online as either electronic books or as

(D) works, and selling them online as either electronic books or as

(E) works, and it will sell them online either as electronic books or

答案是A

对E的解释说 and 前有逗号，于是and it...只能跟主句并列，除非去掉逗号才能跟从句里的内容并列，这个怎么理解？这个逗号的作用是什么？

请指教，多谢~