某船在静水中划行的速率为5m/s，要渡过40m宽的河，河水的流速为3m/s，以下说法中正确的是（ ）
A．该船渡河所用最短时间为10 s
B．该船可能沿垂直河岸的航线抵达对岸，所用时间为10 s
C．如果河水的流速为6m/s，则船也可能沿垂直河岸的航线抵达对岸
D．如果河水的流速为6m/s，则该船渡河所用最短时间大于8 s
答案：B
当船头正对河岸航行时，渡河时间最短，所以该船渡河所用最短时间为
，故A错误
该船可能沿垂直河岸的航线抵达对岸，合速度
，所用时间为
10 s，故B正确
只要船速大于水速，船就可能垂直达到对岸，所以河水的流速为6m/s，船不可能沿垂直河岸的航线抵达对岸，故C错误
船的最短渡河时间，只与河宽和船速有关系，与水流速无关，所以该船渡河所用最短时间依然为10s，故D错误
故选B
Passage One
| It might appear to any casual visitor who may have taken a few rides about town in a taxicab that all New Yorkers are filled with a loudmouthed iii will towards each other. The fact of the , matter is, though, that however cold and cruel things seem on the surface, there has never been a society of people in all history with so much compassion for its fellow man. It clothes, feeds, and houses 15 percent of its own because 1.26 million people in New York are unable to do it for themselves. You couldn’t call that cold or cruel. Everyone must have seen pictures at least of the great number of poor people who live in New York. And it seems strange, in view of this, that so many people come here seeking their fortune. But if anything about the city’s population is more expressive than the great number of poor people, it’s the great number of rich people. There’s no need to search for buried treasure in New York. The great American dream is out in the open for everyone to see and to reach for. It must be because even those people who can never realistically believe they’ll get rich themselves can still dream about it. And they respond to the hope of getting what they see others having. Their hope alone seems to be enough to sustain them. The woman going into Tiffany’s to buy another diamond pin can pass within ten feet of a man without money enough for lunch. They are oblivious to (不在意) each other. He feels no envy; she no remorse. There’s a disregard for the past in New York that dismays even a 1ot of New Yorkers. It’s true that no one pays much attention to antiquity. The immigrants who came here came for some thing new, and what New York used to be means nothing to them. Their heritage is somewhere else. Old million-dollar buildings are constantly being torn down and replaced by new fifty million dollar ones. In London, Rome, Paris, much of the land has only been built on once in all their long history. In relatively new New York, some lots have already been built on four times. Because strangers only see New Yorkers in move, they leave with the impression that the city is in one great mindless rush to nowhere. They complain that it’s moving too fast, but they don’t notice that it’s getting there first. For better and for worse, New York has been where the rest of the country is going. |
The main idea of the last paragraph is that
A．New York is moving fast and is getting nowhere
B．New York seems to be changing for the worse
C．New York is a leader among American cities
D．New York is often criticized for being in great mindless rush