题干

若变量x,y满足约束条件 {x+2y80x40y3 ,则z=2x+y的最大值等于(  )

A:7

B:8

C:10

D:11

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答案(点此获取答案解析)

C

同类题4

阅读下面短文,根据短文内容,从短文后各题所给的A、B、C、D四个选项中,选择最佳选项。

    The town of Pressure and the town of Pleasure were neighbors but they had nothing in common. Residents(居民) built walls to 1 influence from the other town.

    In Pressure, everyone wanted to be the very best. When women gave birth, they would compete(竞争、比赛) to have the baby with the loudest cry. There was violent competition in every aspect(方面) of life. Because 2 was the symbol(象征) of success. People were always busy making money, with no time for relaxation. Some young people couldn't bear the intensity(紧张) and chose to drink to escape(逃避).

    In Pleasure, the motto(格言) was: If only you like it, do it. People grew up without pressure and could do anything they liked. Children played computer games day and night. At school, teachers didn't care 3 students came or not. Workers might sit around the office all day long drinking coffee and doing 4 .Thanks to the lack(缺乏) of regulations(规章制度), nobody worried about losing their jobs. It was pleasure that mattered. The computers they used were old ones from the town of Pressure.

    Some of the young were addicted(上瘾) to drugs(药物)5 the emptiness(空虚) of their lives. Then, people in the two towns began asking themselves, “What is life 6 ?” But, just before life in the two towns completely7, there came a great person—Mr Reason. He went from door to door, talking with people and 8 advice. People in Pressure learnt to be satisfied with what they had 9 people in Pleasure began to make plans. They 10 the walls between them and built a road to connect the two.

同类题5

根据短文理解,选择正确答案。

    Dolphins (海豚) live in a dark underwater world. It's often impossible to see each other or anything else around them, so sound plays an important role in their survival. To communicate with each other, dolphins produce all kinds of sounds.

    Only other dolphins understand what the sounds mean. Scientists haven't uncovered their secret communication, except for one kind of whistle. It might last less than a second, but this whistle is a big deal. Why? Because these whistles are actually names of dolphins — and every dolphin has one. Scientists call these sounds a “signature whistle.” When other dolphins hear the whistle, they know which dolphin is calling.

    Dolphins often hunt by themselves but still need to stay connected to the group. Since they can't always see each other, dolphins use their signature whistles to check in with other dolphins hundreds of yards away. “In coastal areas, dolphins exchange whistles even when they're a third of a mile apart,” says Greg Campbell, who studies animals. That means dolphins shout out to group members that might be nearly five football fields away.

    What's amazing is who names the baby dolphin. Not the mother. Not an auntie dolphin or another group member. Scientists believe the baby dolphin itself comes up with the signature whistle. Like human babies, a baby dolphin plays with sounds throughout its first year. While testing its sound skills, a baby dolphin is doing something amazing. It's creating or figuring out its signature whistle. How or why it chooses its signature whistle is not clear. Studies show that most of the time the signature whistle is nothing like its mother's or group members' whistles.

    When the baby dolphin is about a year old, its signature whistle is set. It repeats it often so the other dolphins learn to recognize it.

    Deciphering(破译) dolphin names is just the beginning of figuring out what dolphins communicate about. Do they chat about sharks? Discuss the tides? Maybe they even have a name for people. Someday scientists are to decipher the rest of dolphins' communication.