except不是做连词么？

我觉得except在这里应该是介词
when they are cornered 是个状语从句
意思是当他们被逼无奈.

应该选D，except此处是个连词，更多的例句（来自权威词典）：
Except is also a conjunction.
Freddie would tell me nothing about what he was writing, except that it was to be a Christmas play...
弗雷迪丝毫不肯透露他正在写什么，只告诉我那是一个圣诞节剧本。
The log cabin stayed empty, except when we came...
除非我们到来，要不然那间小木屋一直都是空的。
Ida would not speak to him except to answer questions...
除了回答问题，艾达都不和他说话。
Nothing more to do now except wait.
现在只能等待。