In Rigveda, there is a dialogue between sage Vishvamitra and which two rivers?

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

In the Vedic Literature, Sabha and Samiti are called two daughters of which deity?

Which of the Vedas is divided into Black and White?

Which of the following Vedas is divided into two branches as Krishna and Shukla ?

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