Until Antoine Lavoisier proved otherwise in the eighteenth

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


MGMAT suggests "if the two actions are clearly sequenced in the sentence it is not necessary to use past perfect tense"..

in the sentence Until X proved, scientists had believed. Clearly suggests that " scientists believing was the first action" & Antoine proving is the action that followed it"

So do we really need "Had" in this question .