Read the following passage and mark the letter A, B, c, or D on your answer sheet to indicate the correct answer to each of the questions from 31 to 35.

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 shopping centre, have a careful look at the floor that you are walking across!

The word useful in paragraph 3 is closest in meaning to______.