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The Sindhu River is mentioned around 50 times in the Rig Veda, one of the oldest known texts in ancient Indian literature. It is considered one of the most frequently mentioned rivers in the Rig Veda.
Not by name (either Zarathushtra or Zoroaster, the Greek transliteration).
Rig Veda
There are 10 mandalas in the Rig Veda.
The Ganges river has no mention in the Rig Veda.
Rig Veda is the first Veda of the four Vedas.
Rig Veda is the first Veda of the four Vedas.
By word of mouth
Rig Veda
Rig veda [by PRABHAT SIR]
Rig Veda, Athar Veda, Yug Veda, and Sama Veda
I think it was because the Hindusim was more useful then Rig Veda.