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 gadget was strictly, beyond any question, a toy. Not a real, workable device. Except for the way it could work under a man's mental skin.... Because there were few adults in the crowd, and Colonel "Biff" Hawton stood over six feet tall, he could see every detail of the demonstration. The children—and most of the parents—gaped in wide-eyed wonder. Biff Hawton was too sophisticated to be awed. He stayed on because he wanted to find out what the trick was that made the gadget work. "It's all explained right here in your instruction book," the demonstrator said, holding up a garishly printed booklet opened to a four-color diagram. "You all know how magnets pick up things and I bet you even know that the earth itself is one great big magnet—that's why compasses always point north. Well Wave Tapper hangs onto those space Every eye was on him as he put the black table and terminated in a control box in the demonstrator's hand. An indicator light, a switch and a knob appeared to be the only controls. "I turn on the Power Switch, sending a surge of current to the Wave Receptors," he said. The switch clicked and the light blinked on and off with a steady pulse. Then the man began to slowly turn the knob. "A careful touch on the Wave Generator is necessary as we are dealing with the powers of the whole world the air. The demonstrator stepped back and the toy rose higher and higher, bobbing gently on the invisible waves of magnetic force that "Only $17.95," the young man said, putting a large price sign on the table. "For the complete set of the Atomic Wonder, the Space Tapper control box, battery and instruction book ..." At the appearance of the price the children rushed away towards the operating model trains. The demonstrator's words were lost in their noisy passage, and after a moment he sank into a gloomy silence. He put Hawton was the only one left after the crowd had moved on. "Could you tell me how this thing works?" the colonel asked, coming forward. The demonstrator brightened up and picked up one of the toys. "Well, if you will look here, sir...." He opened the hinged top. at random—with a few turns of copper wire. Except for these coils The coils were wired together and other wires ran out through the hole in the bottom of the control box. Biff Hawton turned a very quizzical eye on the gadget and upon the demonstrator who completely ignored this sign of disbelief. "Inside the control box is the battery," the young man said, snapping it open and pointing to an ordinary flashlight battery. "The current goes through the Power Switch and Power Light to the Wave Generator ..." "What you mean to say," Biff broke in, "is that the juice from this fifteen cent battery goes through this coils in the model and absolutely nothing happens. Now tell me what really flies the thing. If I'm going to worth of tin, I want to know what I'm getting." The demonstrator flushed. "I'm sorry, sir," he stammered. "I wasn't trying to hide anything. Like any magic trick this one can't be really demonstrated until it has been purchased." He leaned forward and whispered confidentially. "I'll tell you what I'll do though. This thing is way overpriced and hasn't been moving at all. The manager said I could let them matter how it works. The boys in the shop will get a kick out of it," he tapped the winged rocket on his really —what holds it up?" The demonstrator looked around carefully, then pointed. "Strings!" he through a tiny loop in the ceiling, and back down to my hand—tied to this ring on my finger. When I back up—the model rises. It's as simple as that." "All good illusions are simple," the colonel grunted, tracing the black thread with his eye. "As long as there is plenty of flimflam to distract the viewer." "If you don't have a black table, a black cloth will do," the young man "Wrap it up, my boy, I wasn't born yesterday. I'm an old hand at this kind of thing." Biff Hawton sprang it at the next Thursday-night poker party. The gang were all missile men and they cheered and jeered as he hammed up the introduction. "Let me copy the diagram, Biff, I could use some of those magnetic waves in the new bird!" Only Teddy Kaner caught wise as the flight began. He was an amateur magician and spotted the gimmick at once. He kept silent with professional courtesy, and smiled ironically as the rest of the bunch grew silent one and he had set the scene well. He almost had them believing in the through. When the model had landed and he had switched it off he couldn't stop them from crowding around the table. "A thread!" one of the engineers all laughed along with him. "Too bad," the head project physicist said, "I was hoping that a little out. Let me try a flight with it." "Teddy Kaner first," Biff announced. "He spotted it while you were all watching the flashing lights, only he didn't say anything." Kaner slipped the ring with the black thread over his finger and started to step back. "You have to turn the switch on first," Biff said. "I know," Kaner smiled. "But that's part of illusion—the spiel and the misdirection. I'm going to try this cold first, so I can get it moving up and down smoothly, then go through it with the whole works." ILLUSTRATED BY BREY He moved his hand back smoothly, in a professional manner that drew no attention to it. The model lifted smoothly," Biff said and knotted the broken thread. "Here let me show you how to do it." The thread broke again when Biff tried it, which got a good laugh that made his collar a little warm. Someone mentioned the poker game. lift the model only when the switch was on and two and a half volts flowing through the joke coils. With every time. "I still think it's a screwy idea," to people who would be interested?" the older man asked. I remembered from the Bureau of Standards. Luckily he didn't recognize you spotted from the university." "Then the problem is out of our hands and into theirs. All we have to do now is sit back and wait for results." " What weren't interested when we were hammering on their doors with the proof. We've patented the coils and can prove to anyone that there is a reduction in weight around them when they are operating...." don't know what is causing it. No one can be interested in a thing like that—a fractional weight decrease in a clumsy model, certainly not enough to lift the weight of the generator. consumption, tons of lift and such is going to have time to worry about a crackpot who thinks he has found a minor slip in Newton's laws." "You think they will now?" the young man asked, cracking his knuckles impatiently. "I know they will. The tensile strength of that thread is correctly adjusted lift the model—after a small increment of its weight has been removed by the coils. This is going to bug these men. Nobody is going to ask them to solve the problem or concern themselves with it. But it will nag at them because they know this effect can't possibly exist. They'll see at once that the magnetic-wave theory don't know. But they will all be thinking about it and worrying about it. Someone is going to experiment in his basement—just as a hobby of course—to find the cause of the error. And he or someone else is going to find out what makes those coils work, or maybe a way to improve them!" "And we have the patents...." "Correct. They will be doing the research that will take them out of "And in doing so they will be making us rich—whenever the time comes to manufacture," the young man said cynically. "We'll all be rich, son," the older

<question>:
Why does Biff stay to watch the demonstration?

<options>:
A He wants a job at the toy shop.
B He wants to see if the gadget will go on sale.
C He wants to buy the gadget for his nephew.
D He wants to know the trick to how the gadget works.

<answer>:

D