Which of the following statements is correct?
- AThe concentration of a solution is expressed in $\mathrm{dm^3/mol}$.
- BThe empirical formula of a compound always gives the actual numbers of each type of atom in one molecule.
- CThe molecular formula of a compound always contains more atoms than the empirical formula.
- DThe relative atomic mass of an element is $\frac{\text{the average mass of one atom of the element}}{\frac{1}{12}\text{ the mass of one atom of carbon-12}}$.