SAT grammar help!

SAT grammar help!
A Cuban rumba song often begins with meaningless (syllables with lyrics then evolving either) into those of a known song or are improved by the singer.
--syllables,but its lyrics then either evolve
The baker explained that the custard would be firmer (by allowing it to cool slightly before eating it)
--if allowed to cool slightly before being eaten
During the male courtship ritual of the periodical cicada,a type of insect chiefly famous for its long (hibernation alternate periods of synchronized singing) with periods of silence.
--hibernation,periods of synchronized singing alternate
请问括号里的为何要换成下面的句子?