The correct answer is B. A theory is why something happens and a law is how something happens.
Explanation
A scientific theory is a way of describing why chemical and physical phenomena originate. An example of this can be the theory of Charles Darwin on the Evolution of species. In this theory, Darwin set out to explain why the evolution of species and what influenced this development. On the other hand, a scientific law is a description of how some of the phenomena of nature occur. For example, Isaac Newton's Law of Universal Gravitation was intended to describe the gravity. From the above, it can be inferred that the correct answer is B. A theory is why something happens and a law is how something happens.