As motorways become more and more blocked up with traffic, a new generation on flying cars will be needed to ferry people along skyways. That is the conclusion of engineers from the US space agency and aeronautical firms, who _ future commuters traveling by "skycar". These could look much like the concept skycar shown in the picture, designed by Boeing research and development. However, such vehicles could be some 25 years from appearing on the market. Efforts to build flying vehicles in the past have not been very successful. Such vehicles would not only be expensive and require the skills of a trained pilot to fly, but there are significant engineering challenges involved in developing them. "When you try to combine them you get the worst of both worlds: a very heavy, slow, expensive vehicle that's hard to use," said Mark Moore, head of the personal air vehicle(PAV) division of the vehicle systems program at Nasa's Langley Research Center in Hampton, US. But Boeing is also considering how to police the airways and prevent total pandemonium -if thousands of flying cars enter the skies. "The neat, gee-whiz part is thinking about what the vehicle itself would look like," said Dick Paul, a vice president with Phantom Works, Boeing's research and development arm. "But we're trying to think through all the consequences of what it would take to deploy  a fleet of these." Past proposals to solve this problem have included artificial intelligence systems to prevent collisions between air traffic. Nasa is working on flying vehicles with the initial goal of transforming small plane travel. Small planes are generally costly, loud, and require months of training and lots of money to operate, making flying to work impractical for most people. But within five years, Nasa researchers hope to develop technology for a small plane that can fly out of regional airports, costs less than $100,000(PS55,725), is as quiet as a motorcycle and as simple to operate as a car. Although it would not have any road-driving capabilities, it... When engineers develop the skycars, they have to deal with the following difficulties except  _   . A) how to fly out of regional airports B) how to prevent the disorder of the airways C) how to reduce expenses and the vehicle's weight D) how to fly the skycars to enter skies
D

My grandmother pointed at a picture of a six-year-old me and asked, "Sweetie, do you see that?"  Studying my younger self, I smiled and nodded.     "That's my granddaughter," Grandma said. "She lives in Connecticut. She must be.'.Well, I've forgotten how old she'd be now!" My smile froze and my hands began to shake. My grandmother had Alzheimer's disease. It seemed impossible for her to be unable to look after herself. Gradually, not remembering where she placed her glasses became not remembering where her house was, which became not always remembering my name. My birthday gifts from grandma used to be highly expected; she always knew exactly what to get and her cards would make me smile. Now, my gift from her is really from my father, who buys a present in her name since she doesn't remember when my birthday is.^birthday card without her writing just reminds me that my grandmother isn't the same anymore.     I feel sad about it and I find a great comfort through writing. Writing my memories on paper helps me to deal with what is happening and to see Grandma as the~she truly is. Strangely  enough, recording my sadness reduced~of the pain. I know it's her strength that pulls me through the hard times. So now, when I sit on the chair next to Grandma and know she can't remember my name, I swallow the bitter medicine of disappointment and smile. The author's smile froze because her grandmother  _ A) couldn't remember her B) forgot how old she was C) knew nothing about the picture D) wouldn't live with her anymore
A

A black hole is created when a large star burns out. Like our sun, stars are unbelievably hot furnaces   that burn their own matter as fuel. When most of the fuel is used up, the star begins to die. The death of a star is not a quiet event. First there is a huge explosion. As its outer layer is blasted off into space, the dying star shines as brightly as a billion suns. After the explosion gravity pulls in what's left of the star. As the outside of the star sinks toward the center, the star gets smaller and smaller. The material the star is made of becomes tightly packed together. A star is so dense that a teaspoon of matter from it weighs billion of pounds. The more the star shrinks, the stronger the gravity inside it becomes. Soon the star is very tiny, and the gravity pulling it in is unbelievably strong. In fact, the gravity is so strong that it even pulls light into the star! Since all the light is pulled in, none can go out. The star becomes black when there is no light. Then a black hole is born! That's what we know about black holes. What we don't know is this: What happens inside a black hole after the star has been squeezed into a tiny ball? Does it keep getting smaller and smaller forever? Such a possibility is hard to imagine. But if the black hole doesn't keep shrinking, what happens to it? Some scientists think black holes are like doorways to another world. They say that as the star disappears from our universe, it goes into another universe. In other words a black hole in our universe could turn into a "white hole" in a different universe. As the black hole swallows light, the white hole shines brightly--somewhere else. But where? A different place, perhaps, or a different time -- many years in the past or future. Could you travel through a black hole? Right now, no. Nothing we know of could go into a black hole without being crushed. So far the time being, black hole must remain a mystery. Black holes are a mystery--but that hasn't stopped scientists from dreaming about them. One scientist... When the star begins to die   _  . A) there is no fuel left in it B) its outer layer goes into space first C) a huge explosion will happen D) it doesn't give off light any longer

C