Which of the following statements is correct?
- AIf the relative formula mass of hydrated sodium carbonate, $\text{Na}_2\text{CO}_3 \cdot x\text{H}_2\text{O}$, is 286, then $x$ is 10.
- BPhosphoric(V) acid, $\text{H}_3\text{PO}_4$, has a relative molecular mass of 50.
- CThe relative atomic mass of a sample of an element with isotopes of masses 20 and 22 can only be equal to 21.
- DThe relative atomic mass of an element is the average mass of atoms of isotopes of the element compared with the mass of a hydrogen atom.