Yeah CO. I was never any good with the Periodic Table of Elements. LOL.
It's all good . . . in my job I hear folks calling carbon monoxide (CO) by the name carbon dioxide (CO2) all the time . . . maybe because we hear O2 all the time and know about CO2 cartridges and CO2 in the soda/beer . . . whereas CO doesn't quite roll off the tongue as easily.