Why Do Biologists Refer to the "Theory" of Evolution Instead

Why do biologists refer to the "theory" of evolution instead of the "fact" of evolution?

A) Because evolution is not universally believed.
B) Because it is impossible to scientifically test evolutionary principles.
C) Because the theory of evolution summarizes a whole set of scientific facts.
D) Because only theories that have been proven true may be referred to as "facts."