请问一下OG17 SC 695中的to be和were

鹅皇陛下万岁

这道题在论坛上看到很多大牛讨论过，但是重点主要在Ving和Ved的使用上，但是我个人比较纠结这道题to be和were的选择上。

695. By 1940, the pilot Jacqueline Cochran held seventeen official national and
international speed records, and she earned them at a time when aviation was still
so new for many of the planes she flew to be of dangerously experimental design.
A. and she earned them at a time when aviation was still so new for many of the planes she flew to be
B. earning them at a time that aviation was still so new for many of the planes she flew to be
C. earning these at a time where aviation was still so new that many of the planes she flew were
D. earned at a time in which aviation was still so new such that many of the planes she flew were
E. earned at a time when aviation was still so new that many of the planes she flew were

正确答案是E

og解释了一下为什么选择were不选择to be但是我完全没理解orz
"In addition, the causal relationship between aviation being new and planes being of experimental design is more effectively
communicated by the structure so new that many X were Y, not by the structure so new for many X to be Y"

然后又去外网论坛看了一下有一个人说由于to be是表目的的与这题题意不符所以不能选...我个人觉得to be应该不止这一种用法啊


还希望有人能帮我解惑或者帮我指个帖子去学习一下，感激不尽！