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

<passage>:
the U.S. copyright on this publication was renewed.] Man's family tree was awesome enough to give every galactic race an inferiority complex—but then he tried to climb it! humans, each supposedly originating in strict seclusion on as many ! That's more than a legend—one hell of a lot more!" "It is impressive," admitted Taphetta. "But I find it mildly "Try Emmer," he suggested tiredly. "He'll find you irresistible, and he's even more savage than I am." "That's because you're unique," said Halden. "Outside of your own "Is he?" She smiled enigmatically. "Maybe, in a biological sense. Too much, though. You're just right." that's true of all other creatures, intelligent or not, with the sole exception of mankind. Actually, the four of us here, though it's accidental, very nearly represent the biological spectrum of human development. "Emmer, a Neanderthal type and our archeologist, is around the the physical, to make use of her body so as to gain an advantage—what advantage?—for the children she intended to have. Outside of that, nothing mattered, and for the sake of alloying the lower with the beginning of the scale. I'm from Earth, near the middle, though on And beyond her, toward the far end, is Kelburn, mathematician. There's a corresponding span of fertility. Emmer just misses being able to Besides what? She had a beautiful body that could bear superior children—and they might be his. breed with my kind, but there's a fair chance that I'd be fertile with Emmer's side. Meredith, linguist, is on the other side of the middle. He twisted away. With those thoughts, he was as bad as she was. Were The racial disparity was great and she had provoked him, but it was not completely her fault. Besides.... "You're thinking of Earth," said Halden. "Humans require a certain kind Stone Age aren't related to anything else there. We have to conclude "And so, to account for the unique race that can interbreed across only the human race." "I can't think of a better explanation." Taphetta rearranged his ribbons. "Frankly, no one else is much interested in Man's theories about himself." It was easy to understand the attitude. Man was the most numerous though not always the most advanced—Ribboneers had a civilization as high as anything in the known section of the Milky Way, and there were "We've got new data and are able to interpret it better. The theory is systems on which humans existed prior to space travel, we get a certain pattern. Kelburn can explain it to you." the motion. !" "The adjacency mating principle. I've never seen it demonstrated," murmured Taphetta, flexing his ribbons. "Is that the only era that satisfies the calculations?" Halden flushed the sarcasm wasn't necessary. It was true that Kelburn was the most advanced human type present, but while there were differences, biological and in the scale of intelligence, it wasn't as great as once was thought. Anyway, non-humans weren't trained in the fine distinctions that men made among themselves. And, higher or lower, he was as good a biologist as the other was a mathematician. And there was the matter of training he'd been on several expeditions and ability." Taphetta crinkled politely at the reference to his skill. "I had other plans, but I can't evade professional obligations, and an emergency been inexplicably slow in developing and he wasn't completely aware of his place in the human hierarchy. higher, she was as cruel to herself as she was to him. And yet he evolution had taken a different course, but that he was in no sense less complex than Man. It was a paradox that some biologically higher humans hadn't developed as much as lower races and actually weren't prepared for the multitude of life-forms they'd meet in space. Firmon's "What is it, some kind of toxic condition?" "Then you believe it changed mentally as well as physically, that it's smarter?" "I'd say that, yes. It must be a fairly intelligent creature to be so hard to get rid of. But it can be lured into traps, if the bait's humans. Is there anything else you can tell me about the hypothetical ancestor?" Emmer didn't look like the genius he was—a Neanderthal genius, but nonetheless a real one. In his field, he rated very high. He raised a stubble-flecked cheek from a large thick-fingered paw and ran shaggy hands through shaggier hair. "I can speak with some authority," he rumbled. "I was born on a world we'd give something to know what they're made of. Presumably my world don't have the total capacity that later Man does, and yet we're as advanced in civilization. The difference? It must lie somewhere in the planets we live on and it's hard to say just what it is." atomic—because they were human. That was sometimes a frightening thing for non-humans, that the race stuck together. They weren't actually "Of course," rustled the Ribboneer. "But a lot of money and equipment was required for this expedition. I can't believe that the educational institutions that are backing you did so purely out of intellectual curiosity." "Cultural discoveries," rumbled Emmer. "How did our ancestors live? When a creature is greatly reduced in size, as we are, more than cultural discoveries." "Two hundred thousand years ago, they had an extremely advanced civilization," added Halden. "A faster-than-light drive, and we've achieved that only within the last thousand years." "There may be things we can learn from them in mechanics or physics, but wouldn't you say they were better biologists than anything else?" "To the individual, they are, but it won't matter if you discover as much as you think you will. The difference is this: My terms don't technical advancement that multiplied unceasingly. The race that could improve itself through scientific control of its germ plasm had a start a minimum, but when you do use it, be vague. Don't try to imitate them defeat. This time it had no trouble with the raised section. It leaped and that the bodies aren't flesh." have to identify them as their species. If they're smart enough, marrying you." "Primitive," he said, alternately frozen and thawed, though he knew that, in relation to her, he was advanced. "It's almost a curse, isn't it?" She laughed and took the curse away by primeval Earth." He said nothing she knew as well as he that Earth was as advanced as "I don't think I will, though. We might have children." "Would it be wrong?" he asked. "I'm as intelligent as you. We wouldn't have subhuman monsters." "It would be a step up—for you." Under her calm, there was tension. It had been there as long as he'd known her, but it was closer to the surface now. "Do I have the right to condemn the unborn? Should I make them start lower than I am?" The conflict was not new nor confined to them. In one form or another, it governed personal relations between races that were united against non-humans, but held sharp distinctions themselves. "I haven't asked you to marry me," he said bluntly. "Because you're afraid I'd refuse." It was true no one asked a member of a higher race to enter a "Love," she said gloomily. "Physical attraction. But I can't let it lead me astray." "Why not make a play for Kelburn? If you're going to be scientific about it, he'd give you children of the higher type." "Kelburn." It didn't sound like a name, the way she said it. "I don't like him and he wouldn't marry me." "He wouldn't, but he'd give you children if you were humble enough. There's a fifty per cent chance you might conceive."

<question>:
What is the governing principle that classifies the characters in the story?

<options>:
A ancestral bloodline
B physical biology
C galactic prevalence
D intellectual status

<answer>:

A