In which Upanishad is the famous dialogue of Yagnavalkya and Gargi mentioned?

The famous dialogue between Nachiketa and Yama is mentioned in the :

The main theme of Upanishads is –

Which of the following Rig-Vedic Gods is not mentioned in 'Boghazkoi' inscription?

Which of the Vedas is divided into Black and White?

Which of the following rivers mentioned in the Rig Veda indicates the relations of Aryans with Afghanistan?