- Chọn A.

C      +   ${{O}_{2}}$ $\xrightarrow{{{t}^{o}}}$  $C{{O}_{2}}$                      S       +     ${{O}_{2}}$$\xrightarrow{{{t}^{o}}}$   $S{{O}_{2}}$

1$\left( mol \right)$        1$\left( mol \right)$                              1$\left( mol \right)$         1$\left( mol \right)$

 x $\left( mol \right)$ $\to $ $x$$\left( mol \right)$                            y$\left( mol \right)$  $\to $   y $\left( mol \right)$

Từ (1) và (2) ta có:

$\left\{\begin{align}&12x+32y=4,2\\&\frac{x}{2}+\frac{y}{2}={{n}_{{{O}_{2}}}}=0,225 \\ \end{align} \right.$

$\Leftrightarrow \left\{ \begin{align}& x=0,15 \\  & y=0,075 \\ \end{align} \right.$.

C=$\frac{0,15\times 12}{4,2}=42,86%$