Why do guys clenches their jaw when you slap them on the face?

Anonymous
My girl bestfriend had a serious fight with her boyfriend, which happens to be my cousin. He said something he shouldn't have said so she got angry and slapped him on the face. He clenched his jaw and fist. And i'm just curious as to why do guys do that?
Out of anger
Vote A
Some sort of reflex
Vote B
Other (please precise in the comments)
Vote C
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Why do guys clenches their jaw when you slap them on the face?
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