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

<passage>:
throat on the edge of a near-two-dimensional piece of paper. It took valor for them to hunt men in the world of men. In that fact lay a key to their character—if such utterly alien creatures could be said to have character. Extensive research did not uncover any evidence that luxuries, including women. If it only had a lease that could be broken— of the house, which radiated spoke-like from its heptagonal central reminiscent of scores of movie sets of the Deep South. That on the hair. should behave. "Waiting for me?" Tennant asked the girl. She said, "I'd rather be dead. Maybe I am. Maybe we're all dead and this is Hell." tight-fitting tubular gown. "If I could do anything about it...." "Is this crop rotation or did you send for me?" she asked cynically. on with it." 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. It didn't look like a prison—or a cage. Within the dome of the might have been cellophane but wasn't, and it sprouted from a fabric like trees, only their trunks were bark all the way through—except wasn't. It was a prison, a cage. "Okay, I guess," she said. "The way they manage it, there's nothing a female foreman in a garment-cutting shop before being captured and in a harem, even when it's supposedly my own." It arrived before the meal, materializing against one of the seven be it gold, brown or red, an unfamiliar love lyric to a melody whose similarity to a thousand predecessors doomed it to instant success. get through. TV doesn't seem to. Somehow it brings things closer...." foods, with their cigarettes, with everything in their prison—or their cage. Their captors were utterly without a human conception of smell, living, apparently, in a world without odor at all. you want him to fall in love with you? Maybe you're jealous because things are arranged here." "Thanks, Eudalia," said Tennant. "I think I can defend myself. But she's right, Dana. We're as helpless as—laboratory animals. They have the means to make us do whatever they want." all are. We seem to be in a universe of different properties as well as have a hell of a time bringing anyone through alive, and lately they haven't brought anyone through—not alive." "Why do they do it—the other way, I mean?" asked Dana. Tennant shrugged. "I don't know. I've been thinking about it. I suppose it's because they're pretty human." " Human! " Dana was outraged. "Do you call it human to—" "Hold on," he said. "They pass through their gateway to Earth at considerable danger and, probably, expense of some kind. Some of them don't come back. They kill those of us who put up a fight. Those who don't—or can't—they bring back with them. Live or dead, we're just laboratory specimens." "Maybe," Eudalia conceded doubtfully. Then her eyes blazed. "But the things they do—stuffing people, mounting their heads, keeping them on display in their—their whatever they live in. You call that human, Rog?" "Were you ever in a big-game hunter's trophy room?" Tennant asked quietly. "Or in a Museum of Natural History? A zoo? A naturalist's lab? Or even, maybe, photographed as a baby on a bear-skin rug?" "Of course not," he agreed. "In the one instance, we're the hunters, the breeders, the trophy collectors. In the other"—he shrugged—"we're over. He had felt the inner tug of command, said good-by to the women it was As in all of this strange universe, excepting the dome-cages in this even though no reason Opal Opal might extend thousands of yards in iridescent and shot with constantly changing colors. Hence the name Opal. human sense. You will approach without use of your appendages. He went through the entire routine at Opal's bidding. When at last 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 repetition before his workout was done. On Earth, dogs were said to be intellectually two-dimensional creatures. He wondered if they felt this helpless futility when their masters taught them to heel, to point, to Opal was nervous, so much so that he revealed more than he intended. Or perhaps that was his intent his role was to be. He had little time to speculate before Opal seemed to envelop him. He stood before a kidney-shaped object over whose jagged surface 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, smell 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 adolescent's. This hunger was real, not implanted. Everything would be real ... his love for her, the food he ate, the things he touched, his house, his life.... The thought message from Opal crumbled his illusion of freedom. He sank 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. 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 leading another victim to the fourth-dimensional pen. him. The he should have expected a triangle, but somehow he hadn't. And here it was, with all of them going through their paces like a trio of skinned those bodies and removed the heads." So, Tennant thought, they hadn't used the gateway. Not since they had 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 revolting. to do?" "I don't know about you," he said, "but I suspect we're in the same What He lit a cigarette, inhaled. "Relax. I'm not planning revenge. After lover to do something, anything , as long as it was safe. Deliberately, Tennant poured himself a second drink. This might be 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 wanted. 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. They could be hurt, even killed by humans in a three-dimensional world.

<question>:
What is the central theme of the story?

<options>:
A In undesirable circumstances, it is best to remain guarded on the inside, but to display an agreeable, obedient, and non-threatening countenance.
B Be careful what you worship -- be it vanity, reputation, or money -- because if you make it the center of your world, you will always feel inadequate.
C All relationships are ultimately temporal due to three dimensional time and space -- it is only through the fourth dimension that true love can be achieved.
D Experimenting with living creatures -- regardless if it is for entertainment or research -- is unethical, and humans may one day find themselves as subjects.

<answer>:

D