It later became the capital of the Mauryan emperors, in the third century b.c. , and one of the largest cities in the world at 3 km (2 miles) acroseand 12 km (7 miles) along the Ganga.
In the third century B.C., it became the capital chosen by Mauryan emperors.  
In the third century B.C., it didn't become the capital chosen by Mauryan emperors.