What is the passage mainly about

    Since the 19th century, companies have looked for better and cleaner ways of creating electricity. One UK company has created a special type of floor made of square tiles. Under each square, there is a system that can create electricity very cheaply when someone steps on it
    The design of the floor works very well when it is used in very busy areas. And the reason is that a lot of energy is created simply because of the large numbers of people walking across the floor .
    The floor can have other uses too; one of them is to record how many people visit a shopping centre. This kind of information is very useful for shop owners because very often they want to know at which times of days they have the highest numbers of customers. So the next time you visit a bit shopping centre, have a careful look at the floor that you are walking across!
Read the following passage and mark the correct answer : 
1) What is the passage mainly about ?
A . A shopping centre that sells electricity .                        B. A new way to build houses 
C . A special floor that creates electricity .                          D . A difficulty in creating electricity .  
2) The word " useful " in paragraph 3 is closest in meaning to : 
A . kind              B . false                     C.helpful                   D. careful 
3) The word " they " in paragraph 3 refers to : 
A. people           B . times                     C. days                    D . shop owners 
4) Which of the following is NOT mentioned as a good thing about the floor in the passage ?
A . It helps shops attract more customers .                     B . It is good for shop owners. 
C . It creates electricity very cheaply                               D . It works very well in very busy areas .