Read the following passage and questions, then choose the right answer from options, the answer should be one of A, B, C, D.

<passage>:
"But you can't," he told her. "They're too clever." "Is this crop rotation or did you send for me?" she asked cynically. himself—desire that both of them loathed because it was implanted within them by their captors. disapprovingly. "Why take it out on Rog?" she asked bluntly. "He didn't ask to come here any more than we did. He's got a wife back home. Maybe sure of that, though, till we got the radio." "Why haven't they brought more of us through?" Eudalia asked, tamping her close, although neither of them felt desire at the moment. Their captors had seen to that it is." "I'll try not to," he said and stopped, realizing the family party was over. He had felt the inner tug of command, said good-by to the women and returned to his smaller compound within its own barrier dome. Then came the invisible aura of strain in the air, the shimmering illusion of heat that was not heat, that was prelude to his teleportation ... if that were the word. It was neither pleasant nor unpleasant it The captor Tennant called Opal came in through a far corner of the ceiling. He—if it was a he—was not large, although this, Opal might extend thousands of yards in some unseen direction. He had no regular shape and much of him was iridescent and shot with constantly changing colors. Hence the name Opal. Communication was telepathic. Tennant could have yodeled or yelled or sung and Opal would have shown no reaction. Yet Tennant suspected that the captors could hear somewhere along the auditory scale, just as perhaps they could smell, although not in any deep breath. He thought of the space beside Opal. It took about three feet. He was getting good at it. Dog does trick, he thought. He went through the entire routine at Opal's bidding. When at last he was allowed to relax, he wondered, not for the first time, if he weren't mastering some of the alleged Guru arts. At once he felt probing investigation. Opal, like the rest of the captors, was as curious as a cat—or a human being. Tennant sat against a wall, drenched with sweat. There would be endless helpless futility when their masters taught them to heel, to point, to retrieve. Some days later, the training routine was broken. He felt a sudden stir of near-sick excitement as he received the thought: Now you are ready. We are going through at last. Opal was nervous, so much so that he revealed more than he intended. Or perhaps that was his intent Tennant could never be sure. They were going through to Tennant's own dimension. He wondered briefly just what his role was to be. He had little time to speculate before Opal seemed to envelop him. There was the blurring wrench of forced teleportation and they were in another room, a room which ended in a huge irregular passage that might sort of ultradimensional television set, but to Tennant it was as incomprehensible as an oil painting to an animal. Opal was annoyed that Tennant could make nothing of it. Then came the thought: What cover must your body have not to be conspicuous? a costume of mediaeval motley, complete with Pied Piper's flute. He received quick reproof that made his head ring as from a blow. He asked Opal where and when they were going, was informed that he would soon emerge on Earth where he had left it. That told him everything but the date and season. Opal, like the rest of the captors, seemed to have no understanding of time in a human sense. Waiting, Tennant tried not to think of his wife, of the fact that he hadn't seen her in—was it more than a year and a half on Earth? He could have controlled his heartbeat with one of his new powers, but that might have made Opal suspicious. He should be somewhat excited. He allowed himself to be, though he obscured the reasons. He was going to see his wife again ... and maybe he could trick his way into not He shrugged, returned to the feeling of comfort that came from being back here, about to see Agatha again, hold her close in no more than a few minutes. And stay, his mind began to add eagerly, but he pushed the thought down where Opal could not detect it. He took another deep, lung-filling drag on his cigarette, looked around the room that was so important a part of his life. The three women back there would be in a ghastly spot. He felt like a heel for wanting to leave them there, then knew that he would try somehow to get them out. Not, of course, anything that would endanger his remaining with Agatha the only way his captors would get him back would be as a taxidermist's specimen. He realized, shocked and scared, that his thoughts of escape had slipped past his mental censor, and he waited apprehensively for Opal to strike. Nothing happened and he warily relaxed. Opal wasn't tapping his thoughts. Because he felt sure of his captive ... or because he couldn't on Earth? in the cellar rumpus room, where its bleached modernity wouldn't clash with the casual antiquity of the living room. Agatha would complain, naturally, but his being back would make up for house, his life.... Your wife and a man are approaching the house. The thought message from Opal crumbled his illusion of freedom. He sank down in a chair, trying to refuse to listen to the rest of the command: You are to bring the man through the gateway with you. We want another live male. Tennant shook his head, stiff and defiant in his chair. The punishment, when it came, was more humiliating than a slap across a dog's snout. Opal had been too interested in the next lab specimen to bother about his thoughts—that was why he had been free to think of escape. Tennant closed his eyes, willed himself to the front window. Now that But there was no pleasure in it, only a confirmation of his captor's power over him. He was not free of them. He understood all too well what they wanted him to do , the ex-halfback whose bulk was beginning to get out of hand, but whose inherent aggressive grace had not yet deserted him. The man Roger! found that he suddenly didn't care. She couldn't say it was loneliness. Women have waited longer than eighteen months. He would have if his captors had let him. brought the four of them through, not since they had begun to train him desperately for six years, and he no longer wanted her. He was acutely conscious of her perfume. It wrapped them both like an exotic blanket, He needed no telepathic powers to read the thoughts around him then. He heard Agatha's quick intake of breath, saw the split-second look she exchanged with Cass. He turned away, knowing that she was imploring her easier and pleasanter than he had expected. They deserved some of the suffering he had had and there was a chance that they might get it. Tennant knew now why he was the only male human the captors had been able to take alive. Apparently, thanks to the rain-slick road, he had had been sitting there, unconscious, ripe fruit on their doorstep. They had simply picked him up. Otherwise, apparently, men were next to impossible for them to capture. All they could do was kill them and bring back their heads and hides as trophies. With women it was different—perhaps the captors' weapons, whatever they were, worked more efficiently on females. A difference in body chemistry or psychology, perhaps. More than once, during his long training with Opal, Tennant had sent questing thoughts toward his captor, asking why they didn't simply set up the gateway in some town or city and take as many humans as they could understand, it had been like asking an African pygmy, armed with a blowgun, to set up shop in the midst of a herd of wild elephants. It simply wasn't feasible—and furthermore he derived an impression of the tenuosity as well as the immovability of the gateway itself.

<question>:
Why had Roger been trained by Opal?

<options>:
A because Opal wanted to further his experiment
B because Opal needed help building a new gateway
C because Opal was looking for someone to take his place
D because Opal was unable to bring other men back

<answer>:

D