Which of these fits gap 24?

Directions: Complete this passage by selecting the most suitable option for the corresponding gap.

Yawning is an involuntary ______ (21) ______ in which your mouth opens up wide and you inhale deeply followed by exhaling. When you say involuntary, this means that you have no _____ (22) ____ over this action much like other reflexes such as blinking or sneezing. Yawns are not _____ (23) ______ only to human beings as all other animals do this. Although there is no _____ (24) _____ proof as to why we yawn, some researchers have a theory that yawning is a process used to cool our brains off.