Read the following passage and mark the letter A, B, C or D to indicate the correct answer to each of the questions from 21 to 25.

Computers are gradually replacing human labour in many areas of our lives. An important area in computer technology is the use of computer systems to perform jobs which require intelligence, such as learning how to sing or spotting familiar objects. Most computers cannot do these things because they do not have a "brain" to think. However, things are changing as people come up with newer and smarter computers.

Computer scientists have come up with computers which can "think". These special computers do not actually have "brains" but are fed with many facts and pieces of information about the job they are built to do. For example, there are computers which can identify which area contains gold or other materials. They learn about the important qualities of the mineral such as its colour, weight and the areas it is most likely to be found. This makes it easier for the computers to spot the mineral in different parts of the world.

Computers can also be used in factories. Businessmen and factory owners use computers in the form of robots. These robots contain computer systems that enable them to perform the job they are programmed to do. A single robot can do a job which would have take three people to do; this saves businessmen precious time. There would no longer be any need to train workers for different tasks and it would be cost-effective for employers as well.

In time to come, it is possible that computer systems would take away many jobs. However, there is also an increased need for people who can build, design and repair computers. Thus, not all jobs will run out in the near future.

According to paragraph 1, what is one task which needs "thinking" that computers are not able to do?

As long as they ________ for more than three days, they can sleep at my flat.