P Block Elements

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