Which species have delocalized $\pi$ electrons?

I.     ${\text{C}}{{\text{H}}_{\text{3}}}{\text{COC}}{{\text{H}}_{\text{3}}}$

II.     ${\text{NO}}_{\text{2}}^ -$

III.     ${\text{CO}}_{\text{3}}^{2 - }$

A.     I and II only

B.     I and III only

C.     II and III only

D.     I, II and III

C

Although 70% of the candidates gave the expected answer, C, there is minimal delocalization in ethanoic acid, so both C and D were accepted (giving an 86% success rate on the question).