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

<passage>:
got Four, he's happy. Why should he make himself unhappy? As a favor ain't gonna do. "That's the problem. All we got to do is figure out the answer. No use making a pircuit, because a puzzle circuit is just a miniature computer with the solution built in if you can build the pircuit, you've this computer won't work. It won't even do simple arithmetic! Where's Four? He's the only one who understands this thing." Inside the big, central cabin, Grampa waved his pircuit irately in the Grampa was a white-haired 90-year-old who could still go a fast round "Now, Grampa," Fred soothed, but his face was concerned. Fred, once called Young Fred, was Grampa's only son. He was sixty and his hair had Grampa started to say something, but an 8-year-old boy looked up from Junior had won her was, to Grampa, the most hopeful thing he had ever Grampa was still waving his puzzle circuit indignantly. "See!" The "Anybody talks about gravitational pull," Grampa said, snorting, "Go back to your games, Grampa," Fred said impatiently. "We've got work Grampa knitted his bushy, white eyebrows and petulantly pushed the last "That's right, Four," Grampa muttered, "so I did. But whose money "You bought it, Grampa," Fred said. "You, Grampa," Fred said. "You bet! And who made one hundred million dollars out of it that the "And who spent it all trying to invent perpetual motion machines and longevity pills," Joyce said bitterly, "and fixed it so we'd have to go searching for uranium and habitable worlds all through this deadly galaxy? You, Grampa!" "Well, now," Grampa protested, "I got a little put away yet. You'll be "You're never going to die, Grampa," Joyce said harshly. "Just "Well, now," said Grampa, blinking, "how'd you find out about that? computer where Grampa's chair stood. "You brought that pircuit from Grampa looked up, obviously relieved to drop his act of intense "Don't be a kibitzer!" Grampa snapped. "When I need help, I'll ask for it. No dad-blamed machine is gonna outthink Grampa!" He snorted The thing was a featureless blob, a two-foot sphere of raspberry Joyce stood up indignantly. "Well! And don't call me 'Grammy!' It makes at Grampa. "If you'd rather have that blob than me—well!" She swept Grampa glanced warily at Joyce's door to make sure it was completely "Why, thanks, Grampa," Reba said, surprised. "I like you, too, Grampa. If you'd been a few years younger, Junior "You bet he would!" Grampa leaned back and cackled. Then he leaned and to himself, too." Reba smiled at Grampa brilliantly. "And maybe I Grampa turned red and looked quickly toward Four. The boy was staring "That's the idea, Four. You'll be a Peppergrass yet. How about building me a pircuit?" "You get the other one figured out?" "It was easy," Grampa said breezily, "once you understood the principle. The player who moved second could always win if he used the right strategy. Dividing the thirteen lights into three sections of four each—" "That's right," Four agreed. "I can make you a new one by cannibalizing the other pircuit, but I'll need a few extra parts." Grampa pushed the wall beside his chair and a drawer slid out of it. "What is that stuff you drink, Grampa?" Four asked. "Tonic, boy. Keeps me young and frisky. Now about that pircuit—" "Did you ever work on Niccolò Tartaglia's puzzle about the three lovely brides, the three jealous husbands, the river and the two-passenger rowboat?" "Yep," Grampa said. "Too easy." Four thought a moment. "There's a modern variation with three "Sounds good, boy," Grampa said eagerly. "Whip it up for me." paused at Grampa's feet. Grampa reached down to pat it. For an instant, his hand disappeared As if in response, the flivver vibrated. Grampa looked querulously rich. We were going to find radioactives and retire to Earth like billionaires. And all we've done is spent a year of our lives in this glared venomously at Grampa. "It's these '23 models," Grampa put in disgustedly. "They never were Grampa looked up from his pircuit and said, "If I were you, Junior, I Grampa snorted. "Broken-shmoken. Nothing to break, Young Fred. Just a about polarization itself. How does it work, Grampa?" Grampa gave him a sarcastic grin. "Now you're curious, eh? Couldn't be bothered with Grampa's invention before. Oh, no! Too busy. Accept "Let's not get up on any pulpits," Fred growled. "Come on, Grampa, Grampa looked at the four faces staring at him hopefully and the longer. "That's just nonsense! You all know it. Grampa's no genius. He's just a tinkerer. One day he happened to tinker out the polarizer. He doesn't know how it works any more than I do." "Now wait a minute!" Grampa protested. "That's not fair. Maybe I didn't figure out the theory myself, but I read everything the scientists ever wrote about it. Wanted to know myself what made the blamed thing work. What I told you is what the scientists said, near as I remember. Now me—I'm like Edison. I do it and let everybody else worry over 'why.'" "And then you spent everything you got from it on those fool perpetual-motion machines and those crazy longevity schemes when any moron would know they were impossible." Grampa squinted at her sagely. "That's what they said about the gravity "But you don't really know why it works," Junior persisted. "Well, no," Grampa admitted. "Actually I was just fiddling around with "Well, now," Grampa said in bafflement, "it rightly should, you know." "I like your spunk, Reb," Grampa muttered. "He's why the flivver won't work. What Grampa invented was a linear The land of the Fweep rotated once on its axis, and Grampa lowered "Not now, Grampa," Four said inattentively as he watched Fweep making enriched uranium from the pile to put him over the critical mass." "I don't figure it's a whim," Grampa said. "Circular gravity is what creatures." "But he must have some way of reproduction," Reba argued. "An egg or something. He couldn't just have sprung into being as he is now." "I thank him for nothing," Joyce snapped. "He lured us down here by Grampa lowered his bottle and smacked his lips. "Well, boy," he said to "Give her one for me!" Grampa called. "Makes you think, doesn't it?" Grampa said, looking at Fweep benignly. "Less than some I've known," Grampa conceded. "Well, now," Grampa said maliciously, his eye on Joyce, "that's no "Joyce!" Reba cried, horrified. "Grampa was joking, but you actually Grampa lowered the nippled bottle from his lips and chortled. "Junior, "I take it all back," said Grampa. "That computer won't help you with this problem, Junior. This ain't a long, complicated calculation it's a simple problem in logic. It's a pircuit problem, like the one about

<question>:
How did Grampa get rich?

<options>:
A investing in longevity technology
B investing in perpetual motion technology
C inventing space travel technology
D inventing puzzle circuits

<answer>:

C