According to the passage, which aspect of skyscrapers were some residents of North America concerned with in the late 1960's?

• A、The poor reception of radio and TV signals.
  
  

• B、The removal of trees and grass from building sites.
  
  

• C、The harmful effects on the city's plans.
  
  

• D、The obstruction of air traffic
  
  

21.Singapore is a small state in the southeast of Asia           

22.According to the constitution of Singapore, the president of the state is selected           

23.Compared with people in other Asian countries, the Singapore           

24.Several technical colleges, and a teachers college are part of           

25.Though small, Singapore has an industry of its own and can           

A.live a better life

B.made up of more than 50 islands

C.Higher education institute

D.not by the Parliament, but by the people

E.produce goods of various kinds

F.have a big population

The main purpose of the passage is to___________ 

• A、discuss the advantages and disadvantages of skyscrapers
  
  

• B、compare skyscrapers with other modern structures
  
  

• C、describe skyscrapers and their effect on the environment
  
  

• D、illustrate various architectural designs of skyscrapers