63. In June of 1989, Princeton Township approved a developer’s plans to build 300 houses on a large portion of the 210-acre site of the Battle of Princeton, one of only eight Revolutionary War battlefields that had remained undeveloped.

(A) one of only eight Revolutionary War battlefields that had remained undeveloped

(B) one of eight of the only Revolutionary War battlefields that have remained undeveloped

(C) one of the only eight undeveloped Revolutionary War battlefields that remains

(D) only one of eight Revolutionary War battlefields to remain undeveloped (E) only one of the eight remaining undeveloped Revolutionary War battlefields 答案选A，为什么答案用过去完成时呀？

表示在1989，6月前是 remain undeveloped, 之后就不是了。

remain undeveloped 这个动作是在approved 之前的。过去（approved)的过去

(remain）所以就用过去完成时了。