问题
选择题
已知a，b∈R，则“b≥0”是“a2+b≥0”的（ ）
A．充分不必要条件
B．必要不充分条件
C．充分必要条件
D．既不充分也不必要条件
答案
当b≥0时，a2+b≥0成立．
当a=3，b=-1时，满足a2+b≥0成立，但b≥0不成立．
∴“b≥0”是“a2+b≥0”充分不必要条件．
故选：A．
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已知a，b∈R，则“b≥0”是“a2+b≥0”的（ ）
A．充分不必要条件
B．必要不充分条件
C．充分必要条件
D．既不充分也不必要条件
当b≥0时，a2+b≥0成立．
当a=3，b=-1时，满足a2+b≥0成立，但b≥0不成立．
∴“b≥0”是“a2+b≥0”充分不必要条件．
故选：A．
In Britain, people have different attitudes to the police. Most people generally ____them and the job they do，although there are certain people who do not believe that the police____have the power that they do. What does a policeman actually do? It is not ____job to describe. After all, a policeman has a number of jobs in ____.A policeman often has to control traffic, either____ foot in the centre of a town, or in a police car on the roads. Indeed, in Britain, he might be in the Traffic Police and spend all, or a lot of, his time ____up and down main roads and motorways. A traffic policeman has to help keep the traffic moving, stop ____motorists and help when there is an accident. A policeman has to help keep the ___， too. If there is a fight or some other disturbance, we ___ the police to come and restore order. And they often have to ____ situation at great risk to their own ____.We expect the police to solve crimes, of course, so an ordinary policeman, ____ he is not a detective(侦探)，will often have to help___and arrest criminals. And ___ do we call when there is an emergency—an air crash, a____，a road accident, or a robbery? We call the police.___a policeman has to be ____to face any unpleasant emergency that may happen in the ___ world. The police do an absolutely necessary job, they do it ____ well and I support them, but I do not envy policemen. I do not think that I could____do the job of a policeman.
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