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.

Today, millions of people want to learn or improve their English but it is difficult to find the best method. Is it better to study in Britain or America or to study in your own country?

The advantages of going to Britain seem obvious. Firstly, you will be able to listen to the language all the time. You will be surrounded completely by the language wherever you go. Another advantage is that you have to speak the language if you are with other people. At home, it is always possible to speak Vietnamese if you want to and the learning is slower.

On the other hand, there are also advantages to staying at home to study. You don’t have to make big changes to your life. As well as this, it is also a lot cheaper than going to Britain but it is never possible to achieve the results of living in the UK. If you have a good teacher, I think you can learn in a more concentrated way than being in Britain without going to a school.

So, in conclusion, I think that if you have enough time and enough money, the best choice is to spend some time in the UK. This is simply not possible for most people, so being at home is the only viable option. The most important thing to do in this situation is to maximise your opportunities: to speak only English in class and to try to use English whenever possible outside the class.

What is probably the topic of the passage?

The underlined word "viable" in the passage probably means ________.