I just transfered from F-1 to H1-B, and plan to visit china recently. Now, I have two I-94 cards. One is issued at the US custom when I returned to US from China last time as a F-1 student. Another is issued by the BCIS with H-1B approval form 797. Should I return both of these I-94 cards to the airline company before I leave USA, or just one of them? If only one of them, which one I should return? Any information or advice are highly appreciated.