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

<passage>:
came from black space, carrying weird weapons—to fight for a race of which they had never heard. Ro moved cautiously. He knew the jungles of Mars well, knew the swamp snake. Their clear red skin had become blotched and purple, their eyeballs popped, their faces swollen by the poison that raced through their veins. And Ro had seen the bones of luckless men vomited from the mouths of the Droo, the cannibal plants. And others there had been, some friends of his, who had become game for beasts of prey, or been swallowed by hungry, sucking pools of quicksand. No, the jungles of thatched huts, the strange people who lived in these huts instead of in caves, and wore coverings on their feet and shining rings in their ears. And having quenched his curiosity about these people and their For a long moment Ro studied the approaching creatures. Then he gasped in surprise. The pursued was a young woman, a woman he knew. Na! The pursuer was a squat, ugly rat man, one of the vicious Oan who lived in the cliffs. Ro exclaimed his surprise, then his rage. His handsome face was grim as missile, he ran forward. At great speed, he closed the gap between him and the approaching figures. He could see the rat man plainly now—his fanged, frothy mouth furry face and twitching tail. The Oan, however, was too intent The Oan squealed in terror and tried to swerve from his course. The cry. He had seen many Oan die because of the strength and accuracy in came to be playing tag with the Oan." "I was caught in the forest below the cliffs. The Oan spied me and I "They were strange men indeed white as the foam on water, and clothed in strange garb from the neck down, even to coverings on their feet. They made signs of peace—with one hand only, for they carried weapons of a sort in the other. And the men of our tribe made the words. The white men spoke with their thoughts. "They came from a place called Earth, they said. And they came in peace. Our men found they could think very hard and answer back with their own thoughts. And there was much talk and happiness, for friendly "I looked into the valley and saw hundreds of Oan. They had captured Surely he didn't fall to the Oan?" the weapons if they become useless. But when I have slain a few Oan, I will set the white ones free. They will help me to make more weapons. Together we will fight the rat men." Na smiled. Ro was angry, but anger did not make him blind. He would make a good mate. to place. There were no fires, for the Oan were more beast than man and feared another's head was covered with a thatch like a cap of snow on a mountain peak. "You say they came from a place called Earth?" Ro asked Na in wonder. "They traveled through space in their 'ship,'" Na answered. "They called themselves an expedition." good weight and would make short work of any Oan who was foolish enough the presence of many Oan close by. The hair at the base of his neck Suddenly he halted. A dim figure approached. It was one of the Oan, a guard. He was coming straight at Ro. The young Martian shrank back. "The rat men have eyes to cut the night." It was a memory of his mother's voice. She had spoken those words when he was a child, to keep him from straying too far. The Oan was only a few feet away now, but his eyes were not cutting the night. Ro could see his large ears, hear his twitching tail. In a moment the beast would stumble over him. Like a phantom, Ro arose from his crouch. The rat man was startled, frozen with fear. Ro drove his right arm around. The stone in his hand cracked the Oan's skull like an eggshell. Ro caught the body as it Breathing more easily, Ro moved on. He reached the edge of the small clearing without making a sound. Strewn on the ground were shapeless heaps. They would be the slumbering rat men. Ro suppressed an urge to spring amongst them and slay them as they slept. He lay flat on his stomach and inched his way ahead. It was slow work, told them. "After a short while, we will meet Na." "Who is Na?" asked the girl. "She is the one I have chosen for my mate," Ro answered. The white girl was silent. They traveled quite a distance without the men of my tribe." He stopped walking and stared around him for a landmark. They had Then he gave a glad cry. Squinting ahead he saw an approaching figure. It was—His cry took on a note of alarm. The figure was bent low under the weight of a burden. It was a rat man, and slung across his shoulders was a girl. Ro's body tensed and quivered. A low growl issued from deep in his The Oan saw him coming and straightened, allowing the girl to fall. He straight as he prepared to meet the young Martian's attack. Ro struck his foe head on. They went down in a frenzied bundle of fury. The rat man's tail lashed out to twist around Ro's neck. With frantic strength, Ro tore it away before it could tighten. Ignoring the Oan's slashing teeth, the young Martian pounded heavy Over and over they rolled. The Oan clawed urgently at the Martian's air that would give it life. But Ro's hands were bands of steel, tightening, ever tightening their deadly grip. Then, as suddenly as it had started, it was over. The rat man quivered and lay still. Ro dismounted the limp body. His face wore a wildly triumphant smiled as she recognized Ro. The young Martian breathed a sigh of relief. Na turned her head and saw the body of the rat man. She shuddered. "I was coming down the side of the mountain," she said. "I saw him standing at the foot. The shadows were deceiving. I thought it was you. the Oan." "But there are only six of us," one of the white men protested. "There are hundreds of the beasts. We wouldn't have a chance." Ro smiled. their minds?" It was the old man who answered. "We come from a place called Earth, many millions of miles away Ro introduced himself and Na. "What manner of a place is this Earth?" he asked, after the formalities. "Our part of Earth, America, is a great country. Our cities are built What year is it?" Mars, and clusters of wild berries and grapes. table if you wander off. Your next meeting with the Oan may not be so and spoke. It was Carlson who asked, "How do you expect the six of us to attack the rat men?" "The Oan are cowards," Ro answered. "They are brave only because they have your weapons. But now that you are free, you can make more of these sticks that shoot fire." You refuse to help me avenge my people because you are more of a coward than the Oan." "Perhaps the attraction you seem to hold for the Oan can be put to good use," he said aloud. "The sphere is a distance away from the Oan camp. All of the rat men cannot be guarding it. Perhaps, by revealing yourself, you can lure the guards away from their post." He repeated his plan to the others. "She will be a woman alone," said Ro. "The Oan prefer to capture women Ro laughed. "Do you think I will let her go alone? I will be close by. Na can lead the rat men through a narrow part of the valley. I will be above on the cliffs, waiting to pelt them with stones. Carlson or Grimm can be with me to roll an avalanche of rocks on their heads. "This is no time for fighting," he said. "When the Oan are defeated you "Come on," Ro said. "If we spend the rest of the morning here, the Oan over the narrowest part of the valley. If all went well, the Oan would speak of 'legal' or 'ceremony.' After all, it's a personal matter. Who

<question>:
Who or what is an Oan?

<options>:
A The name of the human's fire weapons.
B The name of the red people.
C The name of the human's ship.
D The name of the rat people.

<answer>:

D