The correct order of acidic strength is :

1. ${\mathrm{Cl}}_2{\mathrm{O}}_7>{\mathrm{SO}}_2>{\mathrm{P}}_4{\mathrm{O}}_{10}$                   

2. ${\mathrm{CO}}_2>{\mathrm{N}}_2{\mathrm{O}}_5>{\mathrm{SO}}_3$

3. ${\mathrm{Na}}_2\mathrm{O}>\mathrm{MgO}>{\mathrm{Al}}_2{\mathrm{O}}_3$                   

4. ${\mathrm{K}}_2\mathrm{O}>\mathrm{CaO}>\mathrm{MgO}$

Which is the least polarizable gas among the following?

The correct statement about hydrolysis of ${\mathrm{BCl}}_3$ and ${\mathrm{NCl}}_3$ is-