What is ethene's relative molecular mass, $M_r$?
- Athe average mass of the isotopes of C and H compared to $\frac{1}{12}$ of the mass of an atom of $^{12}\text{C}$
- Bthe atomic numbers of the isotopes of C and H compared to $\frac{1}{12}$ of the mass of an atom of $^{12}\text{C}$
- Ctwice the $A_r$ of C plus four times the $A_r$ of H
- Dtwice the $A_r$ of C plus six times the $A_r$ of H