Which of the following is a characteristic of Python 3?

A key characteristic of Python is its extensibility. The language is designed around a small core with a large standard library and the ability to easily incorporate third-party modules and packages. This modular design allows developers to import existing functionality for a wide range of tasks, from web development to data science, without having to write everything from scratch. This extensibility is a primary reason for Python's popularity and versatility.

A. Python is open source, managed by the Python Software Foundation under a GPL-compatible license, not closed source.
C. Python 3 is famously not backwards compatible with Python 2, which necessitated significant code migration for many projects.
D. Python and Java use different virtual machines and bytecode formats; they are not binary compatible.

1. Python Software Foundation, "About Python™," python.org. The official site states, "Python is a programming language that lets you work quickly and integrate systems more effectively." This integration is achieved through its modular architecture.

2. Lutz, M. (2013). Learning Python, 5th Edition. O'Reilly Media. Chapter 3, "How Python Runs Programs," explains the modular nature of Python and how import statements load other files as modules. (Note: While a commercial book, it is authored by a core figure and used as university courseware).

3. Stanford University, "CS106A: Programming Methodologies," Course Reader. The introduction to Python highlights its "extensive standard library" and the vast ecosystem of third-party packages as key advantages.