Until Antoine Lavoisier proved otherwise in the eighteenth century, many scientists had believed that combustion released phlogiston, an imaginary substance whose properties were not fully understood.

A. many scientists had believed that combustion released phlogiston, an imaginary substance whose properties were not fully understood

B. many scientists believed that phlogiston was an imaginary substance released by combustion and its properties were not fully understood

C. phlogiston was an imaginary substance whose properties were not fully understood and which many scientists had believed was released by combustion

D. phlogiston, an imaginary substance whose properties were not fully understood, was believed by scientists to be released by combustion

E. many scientists had believed that phlogiston was released by combustion and was an imaginary substance whose properties were not fully understood

My Ques: why is “had believed” required int he sentence. I picked B as the correct answer. Please explain