Clive's paper was returned with the answer "barium ll carbonate" marked wrong for the formula #BaCO_3#. Why was his answer wrong?
Typically when a metal shows only one oxidation state the Roman numeral designation is not used.
In general, alkali and alkaline earth metals show only one oxidation state, at least in compounds most chemists commonly deal with, so they do not take Roman numerals. Transition metals typically may choose between multiple oxidation states, so they require Roman numerals.
Here is a bit more help with this concept.