"Fifteen hundred years ago everybody knew the Earth was the center of the universe. Five hundred years ago, everybody knew the E
arth was flat, and fifteen minutes ago, you knew that humans were alone on this planet. Imagine what you’ll know tomorrow.” That's a statement of Agent Kay in from Men in Black. Agent Kay's statement illustrates that theories are A) always true. B) constantly changing. C) describe what happens in the natural world. D) are proven by evidence collected by scientists.
Scientific theories are constantly changing for many reasons. Sometimes it IS because of new evidence. But we would not use the term proven because they ARE always changing! Therefore, the answer is B.
