一道SAT语法题

把句子精缩了去理解.
一、去掉所有附加成分
时间状语 in 1850
同位语 a Black American explorer
处所状语in the mountains of the Sierra Nevada
gateway 的后置定语 to california gold-rush country.
二、留下句子的主要成分
Jim Beckwourth discovered a pass soon becoming an important gateway
其中谓语部分的核心是 discover sth.doing,即 sth.doing 是复合宾语,分词doing 是宾语补足语,意思是发现某物在做某事或即将做某事.由此可见,discovered a pass soon becoming gateway 意思是“发现一条小路很快就要变成重要通道”,这在意思上是说不通的,“要变成重要通道”怎么会被“发现”呢!
应该改为 discovered a pass that/which soon became an important gateway,that 引导的是定语从句,修饰 pass,意思为“发现一条不久变成重要通道的小路”,这才是合情合理的.