Answer
Because carbon only needs to form TWO double bonds with the oxygens, only one 2s and one 2p orbital need to mix to form TWO sp hybrid orbitals on the carbon atom. One sp hybrid orbital forms a sigma bond between C and each O. Pi bonds are then formed between C-O by unhybridized 2p orbitals.
Work Step by Step
Because carbon only needs to form TWO double bonds with the oxygens, only one 2s and one 2p orbital need to mix to form TWO sp hybrid orbitals on the carbon atom. One sp hybrid orbital forms a sigma bond between C and each O. Pi bonds are then formed between C-O by unhybridized 2p orbitals.
