TITLE: The AGT Duality and Beilinson-Drinfeld's Original Formulation of Geometric Langlands Duality via CFT ABS: The original mathematical formulation of geometric Langlands duality by Beilinson-Drinfeld involved CFT, not gauge theory. It was therefore an outstanding question how Kapustin-Witten's gauge-theoretic approach is related to Beilinson-Drinfeld's CFT approach. We will shed light on this question via string/M-theory. In particular, we first consider a modification of our physical setup manifesting the Braverman-Finkelberg geometric Langlands duality to arrive at an AGT duality which relates gauge theory to affine W-algebras. Then, it can be explained that the KW and BD formulations are just string-dual to each other.