【单选题】

In paragraph 2 "leaner variables" and "strategies" are defined by reference to other writers ______.
A．because these writers are authorities in the field and these are recognized as important concepts

B．because these writers are authorities in the field and these are recognized as important definitions
C．because the present author is not sure what these terms mean
D．because the present author wishes to redefine the scope of research in this area