Step 1: Understanding the sequence of Pre-Socratic philosophers.
The sequence of the pre-Socratic philosophers is crucial in understanding the development of early Greek thought:
- Thales (A) is generally considered the first philosopher of Western thought, often known for his belief that water is the fundamental substance of the universe.
- Anaximenes (B) followed Thales, proposing that air, rather than water, was the fundamental substance of the universe.
- Anaximander (C), who came after Anaximenes, introduced the concept of the "Apeiron" (the infinite or boundless), which he believed to be the origin of all things.
- Pythagoras (D) was a later figure, best known for his contributions to mathematics and the concept of number as the essence of reality.
Step 2: Conclusion.
The correct sequence is A, B, C, D, as it follows the chronological order of these philosophers’ contributions.
Final Answer:
\[
\boxed{\text{The correct answer is 2. A, B, C, D.}}
\]