I think it's a because every word has a subject and a verb
Since this is an open question with no choices, I am answering with what I think will best fill in the blank. Note: I do not know exactly what answer they are looking for.
The best answer is: REM
REM is the stage of sleep where dreaming occurs. REM stands for Rapid Eye Movement. Exactly as the title says, REM sleep is characterized by fast back and forth movement of the eyes.
A line of reasoning that presents the opposite side of the author's argument is called (I believe) a counterargument. It states exactly the different thing from what you have first stated.
