The passage discusses the Indus Valley civilisation, which is introduced in Statement 5 as an early urban society, providing necessary context. 5 introduces us to the civilisation as a civilisation from the Bronze Age (old period); and 2 continues on that old period narrative, by stating "...sophistication for its time, this ancient culture...". This creates a clear lexical tie, so 5 should be followed by 2. Statement 2 elaborates on its urban sophistication (planned cities, sanitation, grid layouts). Statement 1 serves as an example of the "architectural prowess", which is mentioned in statement 2. So 21 forms a logical pair. The order 521 therefore goes: General → Specific (civilisation → urban design → examples). Statement 3 then shifts to economic aspects (trade, agriculture), a natural progression from discussing physical cities to how they were sustained. Statement 4 concludes with the civilisation's decline, fitting end as it contrasts its achievements with its mysterious downfall, thereby ending with a thought-provoking note on its decline.The correct order is therefore 52134.