Which of the following statements are correct ?
A. `R - overset(+)(C) = overset(..)(O):` and `R - C -= overset(+)(O):` are resonance structures
B. Methyl carbanion is both isostructural and isoelectronic with ammonia
C. In `overset(4)(C)H_(2)=overset(3)(C)H - overset(2)(C)H=overset(1)(O)`, nucleophilic centre is position 4 while electrophilic centre position 1.
D. `SnCl_(4)` acts as an electrophile
