She (had taught, taught) in an elementary school before she became a professor

[keannu]

In my grammar book, the answer is only "had taught" in this sentence, but isn't "taught" also possible as time sequence is evident due to "before"? Are both correct or only one?
ex)She (had taught, taught) in an elementary school before she became a professor.

 

[5jj]

In my grammar book, the answer is only "had taught" in this sentence, but isn't "taught" also possible as time sequence is evident due to "before"? Yes
Are both correct or only one? Both

When the sequence of events is clear, we frequently use the past simple.