Mauryan Empire
Megasthanes, the ambassador of Selucus at the Mauryan court in Pataliputra, wrote an account of the period in his book

Indika
Both (b) and (c)
Indicoplecusts
Travels of Megasthanes

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Mauryan Empire
Chandragupta Maurya with the help of Chanakya decided to overthrow the Nandas because

Chanakya had been humiliated by the Nandas
Chanakya wanted to restore the ideal of Kshatriya rule
the Nandas were low-born
the Nandas had accumulated a great deal of wealth by extortion and oppression of the people

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