⇔Scientists will continue to investigate the mystery of the yawn. Is it a leftover behavior from before humans had language? Was it a "Hey, I'm sleepy. Let's go to sleep" message? Or is it really just a way to keep our brains cool? While this behavior still is largely a mystery, one thing is certain: you are probably fighting the urge to yawn right about now.⇔

Which line shows the main idea of the bolded paragraph?

A- Scientists will continue to investigate the mystery of the yawn

B- A leftover behavior from before humans had language

C- Was it a "Hey, I'm sleepy. Let's go to sleep" message?

D- You are probably fighting the urge to yawn right about now