$\begin{array}{1 1}(a)\;CH_2^{\delta -}=CH-CH^{\delta +}=O\\(b)\; CH_2^{\delta -}=CH-CH=O^{\delta +}\\(c)\; CH_2^{\delta -}=CH^{\delta +}-CH=O\\(d)\; CH_2^{\delta +}=CH-CH=O^{\delta -}\end{array}$
