GWD16-41 困惑ing

Q41:

The first English settlers to establish a permanent colony in America did it not at Plymouth but along the shore of the James River west of Chesapeake Bay. 

1. it not at Plymouth but
2. so not at Plymouth but
3. not at Plymouth but instead
4. not at Plymouth but did so
5. it not at Plymouth but rather

ANSWERB)

請問大家, 這邊的did so 是只前面establish a permanent colony 的 establish 嗎? 對於這個so的用法最近讓我頗為迷惑  , 其中(A)(D)中的it指代不明可排除,那其他選項各位如何判斷呢?  謝謝回答~~~