In the 1980s advertising revenues accounted for approximately 40% of operating profits of a typical local newspaper; in the 1990s this proportion increased to 57%.
newspaper; in the 1990s this proportion increased to 57%.
newspaper; in the 1990s they increased by 57%.
newspaper, in the 1990s they increased to 57%.
newspaper; increasing to 57% in the 1990s.
newspaper, which increased to 57% in the 1990s.
The explanation for this question states that the sentence is parallel.. In the 1980s must be parallel to in the 1990s? I don't see any parallelism markers here.
Also - Advertising revenues is the subject.. is accounted for the verb?