Amongst the given options, the compound(s) in which all the atoms are in one plane in all the possible conformations (if any), is(are) :

• A

  

• B

  

• C

  ${\mathrm{H}}_2\mathrm{C}=\mathrm{C}=\mathrm{O}$

• D

  ${\mathrm{H}}_2\mathrm{C}=\mathrm{C}={\mathrm{CH}}_2$