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.

It is estimated that about 200 million people use the Internet computer network around the world. The Internet lets people (21)____ at home instead of traveling to work.The Internet allows businesses to communicate with customers and workers in any part of the world for the cost of (22)______ local telephone call.Email allows (23)____ to send documents, pictures and other data from one part of the world to another in at least 5 minutes.People can use the Internet to do shopping, (24)____ saves a lot of time.It is possible to use the Internet for (25)____ - students may connect with their teachers from home to send or receive e.mail or talk their problems through "online" rather than attend a class.

What does the word "This" refer to?