I appreciate Jmimi's reply.
My father left US before his B-2 extension approved. He didn't return anything to the airline since his original I-94 was in INS for the visa renewal. Now the B-2 extension got approved and I need to know where to return the I-94. Today I made couple calls. CIS help line told me to send it back to a US Consule in China. Airport INS told me to send it to a field (local) INS office. I was totally confused. Would you, experienced or knowledgable on this matter, please advise me how to do it. Thank you very much.