已知集合A={x|x2-1=0}，则下列式子表示正确的有（ ）
①1∈A；②{-1}∈A；③∅⊆A；④{1，-1}⊆A．
A．1个
B．2个
C．3个
D．4个
因为A={x|x2-1=0}，
∴A={-1，1}
对于①1∈A显然正确；
对于②{-1}∈A，是集合与集合之间的关系，显然用∈不对；
对③∅⊆A，根据集合与集合之间的关系易知正确；
对④{1，-1}⊆A．同上可知正确．
故选C．
| 完形填空。 | ||||
| The Olympic Games began more than 1 in Greece. At that time Greece was divided (分开) into several states. The people of the states 2 each other; but during the games all wars 3 and the enemies lived 4 . The first Olympic Games were held in 776 BC. They 5 for a thousand years and then were stopped. In 1896,1,500 years 6 , the Olympic Games began 7 . That year the people of many 8 met and decided to form an Olympic Games committee (委员会).The first 9 Olympic Games were held in Athens later that same year. In the first Olympic Games women 10 to take part in them, 11 there are many sporting events for women also. The main sports 12 since the first Olympic Games are running, jumping, throwing, boxing and 13 . 14 of the sports that have been added are football, basketball, shooting and so on. Winter sports were added 15 the Olympic Games in 1924. The Olympic Symbol is five color rings. The rings are connected to each other to show the sporting friendship of peoples. The rings also symbolize (象征 ) the five continents: Europe, Asia, Australia, Africa and 16 . 17 ring has a different color: blue, yellow, black, green and red. All the modern Olympic Games begin officially when 18 arrives carrying a lighted torch from the temple of Zeus to the Olympic stadium. It is considered a 19 of the spirit of good sportsmanship. It also reminds each sports man that he must do his best to be worthy of the ideals of the Olympic-the important thing in the Olympic Games is not winning but 20 . | ||||
|