单选题She did not come back home after midnight, ______ turned her parents very upset and angry. A. which B. that C. who D. what

单选题 ______ is common with productive directors, besides these two movies, he is handling a third one in the meantime.

单选题Both Boeing and Airbus have trumpeted the efficiency of their newest aircraft, the 787 and A350 respectively. Their clever designs and lightweight composites certainly make a difference. But a group of researchers at Stanford University, led by Ilan Kroo, has suggested that airlines could take a more naturalistic approach to cutting jet-fuel use, and it would not require them to buy new aircraft. The answer, says Dr. Kroo, lies with birds. Since 1914, and a seminal paper by a German researcher called Carl Wieselsberger, scientists have known that birds flying in formation—a V-shape, echelon or otherwise—expend less energy. The air flowing over a bird's wings curls upwards behind the wingtips, a phenomenon known as upwash. Other birds flying in the upwash experience reduced drag, and spend less energy propelling themselves. Peter Lissaman, an aeronautics expert who was formerly at Caltech and the University of Southern California, has suggested that a formation of 25 birds might enjoy a range increase of 71%. When applied to aircraft, the principles are not substantially different. Dr. Kroo and his team modeled what would happen if three passenger jets departing from Los Angeles, San Francisco and Las Vegas were to rendezvous over Utah, assume an inverted V-formation, occasionally swap places so all could have a turn in the most favourable positions, and proceed to London. They found that the aircraft consumed as much as 15% less fuel (with a concomitant reduction in carbon-dioxide output). Nitrogen-oxide emissions during the cruising portions of the flight fell by around a quarter. There are, of course, kinks to be worked out. One consideration is safety, or at least the perception of it. Would passengers feel comfortable travelling in convoy'? Dr. Kroo points out that the aircraft could he separated by several nautical miles, and would not be in the unnervingly easy groupings favoured by display teams like the Red Arrows. A passenger peering out of the window might not even see the other planes. Whether the separation distances involved would satisfy air-traffic-control regulations is another matter, although a working group at the International Civil Aviation Organization has included the possibility of formation flying in a blueprint for new operational guidelines. It remains to be seen how weather conditions affect the air flows that make formation flight more efficient. In zones of increased turbulence, the planes' wakes will decay more quickly and the effect will diminish. Dr. Kmo says this is one of the areas his team will investigate further. It might also he hard for airlines to coordinate the departure times and destinations of passenger aircraft in a way that would allow them to gain from formation flight. Cargo aircraft, in contrast, might be easier to reschedule, as might routine military flights. As it happens, American's armed forces are on the ease already. Earlier this year the country's Defense Advanced Research Projects Agency announced plans to pay Boeing to investigate formation flight, though the programme has yet to begin. There are reports that some military aircraft flew in formation when they were low on fuel during the Second World War, hut Dr. Lissaman says they are apocryphal. "My father was an RAF pilot and my cousin the skipper of a Lancaster lost over Berlin," he adds. So he should know.

单选题Among the following cities, the wind direction will not change in______.

单选题Your library has ______ books about computer science than ours.

单选题The stadium has been specifically designed as a ______ for European Cup matches.

单选题A good friend is one who will ______ you when you are in trouble.

单选题Which of the following is most appropriate for developing a learner's integrated language skills?

单选题She wanted desperately to turn the flower-painted china ______ on the apple-green door, and go through, but somehow she could not. A. frame B. pole C. pan D. knob

单选题No further discussions _______ , the meeting was brought to an end.

单选题请选出下面划线部分读音不同的选项

单选题Which do you think might be the proper title for this passage?

单选题 More and more advanced farm machines ______, the agricultural production increased tremendously.

单选题Li Mei has ______ the job and she hopes she gets it.

单选题Why did the name "Dr Maertens" became "Dr Martens"?

单选题He had a pain ______ his back.

单选题When Jack was eighteen he ______ going around with a strange set of people and staying out very late.