Which Buddha text has described 16 Mahajanpads of 6th century B.C.?

Champa was the capital of which Mahajanapada?

Which was the first capital of ancient Mahajanapada Magadh?

Where was the first republican system of the world during 6th century B.C. located?

In which century did the first Magadhan Empire rise?

During the period of 16 Mahajanapadas, Mathura was the capital of –