The type of oxide formed by the element among Li, Na,Be,Mg,B\mathrm{Na}, \mathrm{Be}, \mathrm{Mg}, \mathrm{B}Na,Be,Mg,B and Al that has the least atomic radius is :
A2O\mathrm{A}_2\mathrm{O}A2O
AO
A2O3\mathrm{A}_2\mathrm{O}_3A2O3
AO2\mathrm{AO}_2AO2