Why Are You Laughing?

When Pia and Domu watch cartoons, some scenes will cause them to burst out in guffaws of louad laughter. Sine their outburst comes at the same time, I presume they both find the scene funny, and for the same reason. 

But I have my doubts. 

Because in the middle of their outburst, while still laughing, Pia will ask Domu: "Why are you laughing?"

I don't know how to understand her question.  Which one does she mean?

"Why are you laughing?"
"Why are you laughing?"
"Why are you laughing?"
"Why are you laughing?"