Read the passage carefully and choose the correct answer.

 In the world today there are 5,000 to 6,000 living languages, of which English is the most widely used. As a mother tongue, it ranks second only to Chinese, which is little used outside China.

English is the most international of languages. It is used as the language of aviation, international sport and pop music. Sixty percent of the world’s radio stations broadcast in English, and more than half of the world’s scientific papers are printed in English.

It is true that a great number of people are involved in the use of English. To people in Africa, Asia, and South America, English is an important foreign language to master. In most countries in the world, the English language is used as the language of business, commerce, and technology. English is now an effective medium of international communication. However, it is the written English which is not systematically phonetic, that causes difficulties to non-native speakers.

What difficulties may learners of English be confronted with? Why?