Task: Question Generation
Input (English): Background Paragraph: Plates include both oceanic crust and continental crust. Stable subduction zones involve the oceanic lithosphere of one plate sliding beneath the continental or oceanic lithosphere of another plate due to the higher density of the oceanic lithosphere. That is, the subducted lithosphere is always oceanic while the overriding lithosphere may or may not be oceanic. Subduction zones are sites that usually have a high rate of volcanism and earthquakes.[2][better source needed] Furthermore, subduction zones develop belts of deformation and metamorphism in the subducting crust, whose exhumation is part of orogeny and also leads to mountain building in addition to collisional thickening. 
Story: David is interested in geology. One day he was studying about how the earth's crust works. He specifically noticed two lithosphere, lithosphere A an lithosphere B. Lithosphere A is an oceanic lithosphere while lithosphere B is an overriding lithosphere. He also noticed two 
distinct zones, zone A an zone B. Zone A is a subduction zone, but zone B is not a subduction zone. Just reading about geology made David excited. Now, he wants to 
visit places with high volcanic activity. For that he chose place C. To compare with place C geology he would also visit place D, which does not have any volcanic activity.
Output (English): Which lithosphere would always be oceanic, lithosphere A or lithosphere B?