问题
单项选择题
设有关系R，S和T如下图。关系T由关系R和S经过( )操作得到。R
A．A
B．B
C．C
D．1 2 3 4 1 6 3 2 4
答案
参考答案：B
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设有关系R，S和T如下图。关系T由关系R和S经过( )操作得到。R
A．A
B．B
C．C
D．1 2 3 4 1 6 3 2 4
参考答案：B
| 阅读下面短文，从文后方框内所给的ABCDE中，选出恰当的一项还原到文中合适的位置， 使文章意思完整且符合逻辑。 | |
| We sometimes call the lion "the king of the jungle" Because it is stronger and more dangerous than other animals. 1_______ It is the highest animal in the land food chain. In the sea, the shark is one of the animals at the top of the food chain. It can swim faster than most other sea 2_______ 3_______ People are! We are not the biggest or fastest. but we are the cleverest. We could not run faster than cheetahs, so we invented cars and now we can move the fastest. We could not fight better than joins. so we invented puns. Now we ran hunt the best-People's inventions are clever, but they can be dangerous, too. For example, people invented lactones, and now they make the air and rivers dirty. 4_______ Dirty rivers kill fish. Then bears cannot eat the fish and die. If we kill one part of the food chain. other parts also die. Remember that we are a part of the food chain. so we are in danger, too! 5_______ We should and we could do something for the world!
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