求分析名词能不能引导状语从句AB.S. in biology, for example, could allow a s

求分析名词能不能引导状语从句
A
B.S. in biology, for example, could allow a student to find a job
in a lab after graduation, a first step on the road towards a career in
scientific research.
这里的a first step引导的什么从句?同位语从句?是不是省略了which is?(非限制性定语从句不是不可以省略的吗?)
The same major could also be used to apply to medical
school, and a career as a doctor.
这里的a career 如果像上面句子一样,为什么还要用and链接?
这两个句子有什么不同?