With reference to the “Urban Revolution” thesis proposed by Gordon Childe, consider the following statements:

1. The emergence of cities was closely linked to surplus agricultural production.
2. 
   
3. Writing systems developed as an administrative necessity in early urban centres.
4. 
   
5. Urban origin, according to Childe, was primarily driven by maritime trade expansion.
6. 
   
7. Social stratification was a defining feature of early urban settlements.
8. 
   

Which of the statements given above are correct?