賜教OG11-047 comma 相關

正在趕OG11 的SC 可是碰上這題感覺可能自己有點鑽牛角尖, 煩請賜教..

By providing such services as mortgages, home improvement loans, automobile loans, financial advice, and staying within the metropolitan areas, Acme Bank has become one of the most profitable savings banks in the nation.

A) financial advice, and staying

B) financial advice, and by staying

C) and financial advice, staying

D) and financial advice, and staying

E) and financial advice, and by staying

選E

OG explains: the first part of the sentence describes the two reasons for Acme Bank's success; those reasons should be written in two parallel phrases:
   by providing such services as ... advice and by staying within metropolitan areas. When by is dropped, staying seems to be part of the list of services. Staying also appears to be the final elements have preceded it, each correctly separated with a comma, followed by the conjuction and, which makes the reader anticipate a final elemtns.  The list should read: mortgages, home improvement loans, automobile loans, and financial advice.

在思考選擇的時候, 我並沒有問題找出考點 parallelism, 個人選擇了C, 但是E有考慮過但最後捨棄. 原因即為我認為 ,and
  之後應該要跟隨完整句型, 而非跟前段的從句 by providing... 並列. 之所以不選E就是因為如果選項E 沒有了 comma 我就會選. 所以最後選擇了C, 而把第二從句 staying within the metropolitan areas 當作Acme Bank主語的插入定語修飾．即為: By providing..., 定語修飾, 句子一.

感覺OG 總是愛打一些迷糊帳, 一些模菱兩可的潛規則也沒有個精闢解釋. 如同 double/twice..

以下為另外一個有關comma的問題, 煩請NN顯顯神蹟..

http://forum.chasedream.com/dispbbs.asp?BoardID=23&ID=365240&replyID=&skin=1