Metal M has only one oxidation number and forms a compound with the formula \({\text{MC}}{{\text{O}}_{\text{3}}}\). Which formula is correct?

A.     \({\text{MN}}{{\text{O}}_{\text{3}}}\)

B.     \({\text{MN}}{{\text{H}}_{\text{4}}}\)

C.     \({\text{MS}}{{\text{O}}_{\text{4}}}\)

D.     \({\text{MP}}{{\text{O}}_{\text{4}}}\)

C

The question proved surprisingly challenging, as indicated by a high number of blank responses and a difficulty index of 55%. This would seem to indicate that a disturbing number of candidates are not aware of the charges on the common ions. It was however a good discriminator with a discrimination index of 0.55.