Which two ancient civilizations primarily thrived in the Tigris-Euphrates Valley?

The Babylonian and Assyrian civilizations significantly flourished in the Tigris-Euphrates Valley, also known as Mesopotamia. This region is often referred to as the "cradle of civilization" due to its rich agricultural land and early developments in writing, law, and urbanization. The Babylonians, known for their contributions to culture, science, and law, particularly with the code of Hammurabi, established significant cities like Babylon. Meanwhile, the Assyrians built a powerful empire leveraging military conquests, and they are well known for their advancements in architecture and administrative systems.

Sumerians, while also part of ancient Mesopotamia, were distinct from the later Babylonian and Assyrian civilizations and were not mentioned in the correct answer. The other options present civilizations that were either geographically separate from the Tigris-Euphrates Valley or developed later in areas like Egypt or Persia, which do not share the same historical context within that specific valley region.