Read the followingpassage and mark the letter A, B, C or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks.

Today, computer companies sell many different programs for computers. First, there are programs for doing math problems. (21)____________, there are programs for scientific studies. Third, some programs are like fancy typewriters. They are often used by writers and business people. (22)____________ are made for courses in schools and universities. And finally, there are programs for fun. They include word games and puzzles for children and adults.

There are many wonderful computer programs, but there are other reasons to like (23)____________. Some people like the way computers hum and sing when they (24)____________. It is a happy sound, like the sounds of toy and childhood. Computers also have lights and pretty pictures. And computers even seem to have (25)____________. That may sound strange, but computers seem to have feelings. Sometimes they seem happy, sometimes they seem angry like a human being.

What does the word “they” in the passage refer to?