问大家一道新GRE OG上的一道题，望不惜赐教~

lida_UID: 2875855

1. A person who agrees to serve as mediator between two warring factions at the
request of both abandons by so agreeing the right to take sides later. To take sides
at a later point would be to suggest that the earlier presumptive impartiality was
a sham.

The passage above emphasizes which of the following points about mediators?
 A They should try to form no opinions of their own about any issue that is
related to the dispute.
 B They should not agree to serve unless they are committed to maintaining
a stance of impartiality.
 C They should not agree to serve unless they are equally acceptable to all
parties to a dispute.
 D They should feel free to take sides in the dispute right from the start,
provided that they make their biases publicly known.
 E They should reserve the right to abandon their impartiality so as not to be
open to the charge of having been deceitful.

这道题其实凭大概感觉能选出来答案B，但是我很困惑这句话的表达：A person who agrees to serve as mediator between two warring factions at the
request of both abandons by so agreeing the right to take sides later.其主句是A person abandons吗？

linacreUID: 2468672

主句是A person abandons the right to take sides later.

fantasykUID: 2923372

那么by so agreeing是什么意思？这个方式状语好像意思不完整啊