Who Is Associated with the Following Summary of the Economic

Who is associated with the following summary of the economic way of thinking: "The theory of economics does not furnish a body of settled conclusions immediately acceptable to policy.It is a method rather than a doctrine,an apparatus of the mind,a technique of thinking which helps its processor draw correct conclusions."

A) John Maynard Keynes
B) Alfred Marshall
C) Adam Smith
D) President Harry Truman