Which of the following does not exist?

Consider the following reactions.

$$ X+\mathrm{O}_2 \rightarrow \mathrm{Cu}_2 \mathrm{O}+\mathrm{SO}_2, \mathrm{Cu}_2 \mathrm{O}+X \rightarrow \mathrm{Cu}+Y \uparrow $$

The shape of the molecule $Y$ is

In contact process of manufacture of $\mathrm{H}_2 \mathrm{SO}_4$, the arsenic purifier used in the industrial plant contains

Observe the oxides $\mathrm{CO}, \mathrm{B}_2 \mathrm{O}_3, \mathrm{SiO}_2, \mathrm{CO}_2, \mathrm{Al}_2 \mathrm{O}_3$. $\mathrm{PbO}_2, \mathrm{Tl}_2 \mathrm{O}_3$. The number of acidic oxides in the list is