question:  Why had Roger been trained by Opal?
context: Their captors had no idea of how a woven design should behave. He could sense the restless stirring of the woman within Dana, just as he could feel the stirring toward her within himself—desire that both of them loathed because it was implanted within them by their captors. The brook was practically water, but the small stones over which it flowed were of no earthly mineral. They entered the house, which had no roof, continued to move beneath a sky that glowed with light which did not come from a sun or moon. Eudalia, who had borne twin girls recently, was lying back, newly thin and dark of skin and hair, smoking a scentless cigarette. Eudalia had been a female foreman in a garment-cutting shop before being captured and brought through. He turned to Olga, broad, blonde and curiously vital, who sat perfectly still, regarding him over the pregnant swell of her dirndl-clad waist. Olga had been a waitress in a mining town hash-house near Scranton. Tennant wanted to put an encouraging hand on her shoulder, to say something that might cheer her up, for she was by far the youngest of the three female captives, barely nineteen. "But he doesn't have to—have them," objected Olga. In fact, Tennant thought, only her laughter was unpleasant, a shrill, uncontrolled burst of staccato sound that jarred him to his heels. It arrived before the meal, materializing against one of the seven walls of the roofless chamber. Olga sat up straight, her pale blue eyes round with utter disbelief. Dana stood almost in the center of the room, carmine-tipped fingers clasped beneath the swell of her breasts. Somehow it brings things closer...." Tennant caught Dana looking at him and there was near approval in her expression—approval that faded quickly as soon as she caught his gaze upon her.
output:  because Opal was unable to bring other men back
options:  because Opal wanted to further his experiment  because Opal needed help building a new gateway  because Opal was looking for someone to take his place  because Opal was unable to bring other men back