You can be polite without giving any more information. "Did you go to China for business or pleasure?" "I'm sorry, but as a US citizen, my rights enable me to

You can be polite without giving any more information. "Did you go to China for business or pleasure?" "I'm sorry, but as a US citizen, my rights enable me to refuse to answer any questions you ask. I am invoking that right." You can even leave out the "I'm sorry" if you want. You make it clear why you are doing this, which means you get to protect yourself and educate others. You don't have to be a jerk to defend your rights, and I firmly believe you will be more effective (generally) at defending those rights.