\(\begin{array}{l}
{M_C} = {M_{Ca}} \times 0,1 = 40 \times 0,1 = 4\,dvC\\
\Rightarrow C:Heli(He)\\
{M_B} = 4{M_C} = 4 \times 4 = 16\,dvC\\
\Rightarrow B:Oxi(O)\\
{M_A} = 2{M_B} = 2 \times 16 = 32\,dvC\\
\Rightarrow A: \text{ Lưu huỳnh } S
\end{array}\)