# Which of the following is not tetrahedral ?

$\begin {array} {1 1} (a)\;BF_4^{-} & \quad (b)\;NH_4^{+} \\ (c)\;CO^{2-}_{3} & \quad (d)\;SO_4^{2-} \end {array}$

(c) $CO_3^{2-}$