I want to know the answer to the following question:
What are the two main divisions of Iranian tongues?
All the background I'm provided with is that "Confusing the issue is the introduction of a western Iranian substrate in later Avestan compositions and redactions undertaken at the centers of imperial power in western Iran (either in the south-west in Persia, or in the north-west in Nisa/Parthia and Ecbatana/Media).". Is that enough to answer the question?