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问题 选择题

“a2=1”是“直线x+y=0和直线x-ay=0互相垂直”的（　　）

A．充分而不必要条件

B．必要而不充分条件

C．充要条件

D．既不充分也不必要条件

答案

“a2=1”可得a=±1；“直线x+y=0和直线x-ay=0互相垂直”所以a=1，

所以“a2=1”是“直线x+y=0和直线x-ay=0互相垂直”的必要而不充分条件．

故选B．

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                                       World Horticultural Expo 2011
                                        (2011西安世界园艺博览会)
      The 2011 International Horticultural Exposition will be held in 'Xian from April 28th to October 22nd
in the year 2011. 

Chang'an Tower, the highest building in the
expo garden, serves as the symbol of Xi'an Expo.
People can go up to the observation decks (观景台)
at the top and catch the whole view of the expo
grounds and the historic city.
Theme Park is designed to be integrated
with the surrounding dock (码头) and
land terrains (地形). The Theme Pavilion
is the main building that has been specially
built  for the 2011 International
Horticultural  Exposition.
    
The Greenhouse is built mainly of glass
and steel, and it is on Splendid Lake in the
eastern side of the expo park.



     Guangyun Entrance is the main
entrance to the Expo site. It's amazing
because it crosses the 60-meter wide
Expo Avenue and that it can hold as
many as 20,000 passengers during peak
hours (高峰时间).
1. _______, visitors can catch the whole view of the expo grounds and the historic city.
A. On Changan Tower
B. In the Theme Park
C. In the Greenhouse
D. At Guangyun Entrance
2. The Greenhouse is in the _______ side of the expo park.
A. northern
B. southern
C. western
D. eastern
3. Guangyun Entrance can hold about _______ passengers during peak hours.
A. 40,000
B. 30,000
C. 20,000
D. 10,000
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单项选择题

西方园林是以法国为代表的欧洲园林，最早形成于公元前5世纪的（）

A.古罗马

B.古希腊

C.古埃及

D.文艺复兴时期
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