A compound contains equal masses of the elements $A, B$ and $C$ if the atomic weights of $A, B$ and $C$ are 20, 40 and 60 respectively. The empirical formula of the compound is
(A) $A_{3} B_{2} C$
(B) $\mathrm{AB}_{2} \mathrm{C}_{3}$
(C) ABC
(D) $\mathrm{A}_{6} \mathrm{B}_{3} \mathrm{C}_{2}$